Before I begin, please forgive my tardy response. While the Coronavirus has not touched my family, it has touched members of our community and we own and operate a store considered essential. It's been a lot of work and very distracting. Thank you for your patience. (And you should post a complete question here, not just a link back to the old one. Cheers!)
Your question piqued my interest on two levels.
I've been a pastor and had to deal with families suffering abusively manipulative relationships.
While not suffering full-blown NPD, my own father has some of the attributes, which allows me to relate.
My first reaction to your excerpted statement was that it seemed silly, but that's because I absorbed the information "backwards." (*sigh*) I hate imperfection, it's a pain in the neck. As I thought about it more and read through the links you provided, I could comprehend something of the pain the ACoNs suffer and the difficulty they have trying to fulfill that necessary fifth commandment.
To begin, the Bible teaches a simple truth:
If any man come to me, and hate not his father, and mother, and wife, and children, and brethren, and sisters, yea, and his own life also, he cannot be my disciple. (Luke 14:26)
And he said unto them, Verily I say unto you, There is no man that hath left house, or parents, or brethren, or wife, or children, for the kingdom of God’s sake, who shall not receive manifold more in this present time, and in the world to come life everlasting. (Luke 18:29-30)
Christians are specifically required to separate themselves from the ungodly and follow Jesus. At first glance this would seem to be at odds with the commandment to honor our parents (Exodus 20:12), but this isn't so. Let's introduce the difference between forgiveness and consequence.
Of course, society needs to be protected from hardened criminals, because mercy cannot rob justice. Bishop1 [Christopher] Williams addressed this concept so well when he said, “Forgiveness is a source of power. But it does not relieve us of consequences.” When tragedy strikes, we should not respond by seeking personal revenge but rather let justice take its course and then let go. It is not easy to let go and empty our hearts of festering resentment. The Savior has offered to all of us a precious peace through His Atonement, but this can come only as we are willing to cast out negative feelings of anger, spite, or revenge. For all of us who forgive “those who trespass against us,” even those who have committed serious crimes, the Atonement brings a measure of peace and comfort. (Apostle James E. Faust, emphasis mine)
The reason forgiveness vs. consequence must be understood is that the Church of Jesus Christ of Latter-day Saints does not believe mortal humans are on this Earth to suffer. Quite the opposite, we believe that we "are, that [we] might have joy" (2 Nephi 2:25). It is difficult for a child to find joy when that child is not honored and respected by a parent. It is my opinion that this idea is reflected by 2 Corinthians 6:14, which happens to reflect the previous verses in Luke quite well.
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
While Paul originally intended that verse to apply to a married couple, I personally see no difference between that and the parent-child relationship.2 Children are not slaves to their parents. Indeed, we very much believe it is the responsibility of parents to properly train, care for, and raise their children in a Godly manner.
And again, inasmuch as parents have children in Zion, or in any of her stakes which are organized, that teach them not to understand the doctrine of repentance, faith in Christ the Son of the living God, and of baptism and the gift of the Holy Ghost by the laying on of the hands, when eight years old, the sin be upon the heads of the parents. … And they shall also teach their children to pray, and to walk uprightly before the Lord. (Doctrine & Covenants 68:25-28)
Basically, the gospel of Christ is not served if someone as an individual, or even society in general, lives in danger due to an unrepentant person. My job as the victim of a crime or abuse is to forgive, to let the resentment pass, to love (meaning to seek the best welfare of the other, even before my own) — but this does not absolve the perpetrator of the crime.
Consider this, the Lord taught that we should forgive "seventy times seven" (Matt 18:21-22), which is usually interpreted as "always." This is strengthened by Doctrine & Covenants 64:10-11,
I, the Lord, will forgive whom I will forgive, but of you it is required to forgive all men. And ye ought to say in your hearts—let God judge between me and thee, and reward thee according to thy deeds.
But verses 12-14 read,
And him that repenteth not of his sins, and confesseth them not, ye shall bring before the church, and do with him as the scripture saith unto you, either by commandment or by revelation. And this ye shall do that God may be glorified—not because ye forgive not, having not compassion, but that ye may be justified in the eyes of the law, that ye may not offend him who is your lawgiver—verily I say, for this cause ye shall do these things.
The Lord, above all, is no fool and knows that there can and will be situations that are simply untenable. What, then, should happen if the sinner never repents? It is for this cause that the Lord also taught in Doctrine & Covenants 98:39-46 that we are always to forgive people who repent, even if they repent over and over. But if they do not repent, we are to forgive through three transgressions, but on the fourth, no. I strongly recommend taking the time to read those verses, but here's the salient part:
But if he trespass against thee the fourth time thou shalt not forgive him, but shalt bring these testimonies before the Lord; and they shall not be blotted out until he repent and reward thee four-fold in all things wherewith he has trespassed against thee. And if he do this, thou shalt forgive him with all thine heart; and if he do not this, I, the Lord, will avenge thee of thine enemy an hundred-fold; and upon his children, and upon his children’s children of all them that hate me, unto the third and fourth generation.3
Therefore (and with a MASSIVE disclaimer I'm going to make presently), the answer is yes, given that the child has made at least three sincere attempts to forgive and reconcile with the abusive parent, if the parent remains abusive (a transgressor), then the child is not dishonoring the parent by severing the relationship. That is the consequence of the parent's refusal to repent.
This answer, and the question itself, skirts dangerously close to this site's rules about offering pastoral advice (i.e., that we do not). Indeed, if I recall correctly, you had to rewrite your original post to get under that bar. I have provided an answer that reflects LDS theology—but there are two other aspects of that theology that must be considered.
LDS Bishops are common judges in Israel (Doctrine & Covenants 107:72, 74) and are authorized under Priesthood authority to judge a victim-child's actions and decisions in relation to the parents. No decision like this should be taken lightly or without counsel. Telling you "yes" without you realizing that no one can (or should) make that decision alone would be very irresponsible on my part. Obviously, I do not and cannot know enough about any situation you might be aware of to provide a one-size-fits-all answer.
A common question is "why does God allow bad things to happen to good people?" This is a spectacularly good question. One answer is that He doesn't in all cases. For example, when asked if the dead workers under a fallen tower were sinners, the Lord responded (paraphrasing) "No, they died because the tower fell on them" (Luke 13:1-5). Another is that good people are allowed to suffer to ensure judgement both for the good and the evil (Alma 14:8-11). The "right" decision in any situation like that posed in the question is complicated, but it can be thought of this way: if you personally choose to honor your parent, no matter how it hurts you and those around you, and your parent never repents, then you will be blessed in Heaven for your suffering and your parent judged for his/her wickedness. On the other hand, if you follow the Lord's admonition and conclude that it's right to leave, you will be blessed in Heaven for that, too. The Lord respects responsible decisions, but usually doesn't quick (usually selfish) decisions.
Therefore, while LDS theology permits separation in the case you've presented, it does not encourage it. Rather, the Lord has provideded authoritative counsel to help the victim of these circumstances make a responsible decision.4 To liken another scripture unto us… such a person would in no wise lose his/her reward.
Answers to Comment Questions
It appears you're looking for a clear-cut answer to a question that isn't clear-cut. Every situation involving emotional instability and abuse will be different and the presumptive solution will change. For some, the right answer will be to sever (and quickly!). For others, the right answer will be to not sever and bear the burden. For yet others, the right answer will be something in between. This is one of the reasons why counseling with both ecclesiastical leaders and professionals is so important — because in most instances involving narcissism, the answer will have more to do with the emotional, physical, and spiritual stability of the child-victim than it will the abusive parent.
At the risk of wresting the scriptures (2 Peter 3:15-16, Doctrine & Covenants 10:62-63, Alma 41:1), let me speak briefly about love—which is the basis for honoring one's parents.
The Lord commanded something very much contrary to the natural man: He commanded us to love everyone; our neighbors, strangers, even our enemies. Indeed, the apostle Paul taught that brotherly love should continue, suggesting that it's something we should always do. One of the fundamental doctrines of Christ's gospel is that to be disciples, we must love.
But, to use what I think is the simplest case, how does one love an enemy? What does it even mean to love an enemy? There are a lot of definitions of "love" out there and most of them are, frankly, wrong. Love is not sexual intimacy, though such intimacy may be a part of the experience of love. Love is not blind devotion, though having faith in another may also be part of the experience of love.
It is my opinion (and it's important that you understand this, it is my opinion) that love is a choice. Usually, the choice to put the welfare (spiritual, emotional, and physical) of another before your own.
Greater love hath no man than this, that a man lay down his life for his friends. (John 15:13)
But when it comes to the victims of abuse, that choice becomes complicated. Choosing to preserve the relationship doesn't benefit the abuser in any way Christianity would recognize. I could certainly complicate the discussion (what if the victim-child is the bread-winner for the abusive parent? Severing the relationship may have severe consequences for the physical welfare of the parent). But that's why the need for counseling is so great—to be sure the needs of the child are balanced against the honors deserved by the parent in a Christ-centered way.
A student of the Bible would point out that I didn't reference the entire passage from Matthew 5. Here it is:
Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; that ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
For if ye love them which love you, what reward have ye? do not even the publicans the same? And if ye salute your brethren only, what do ye more than others? do not even the publicans so?
Be ye therefore perfect, even as your Father which is in heaven is perfect.
In my opinion, a lot of people have lived their lives filled with unjustified feelings of guilt due to taking that last sentence out of context. It is obviously impossible to be as perfect as a resurrected, sanctified, exalted being like our Heavenly Father until we, ourselves, become resurrected, sanctified, and exalted. But remember the context: It's easy to love people who already love you. You need to learn important skills. You need to train important strengths. You need to learn to love people who do not love you. Doing this, you learn to live your life in a way our Heavenly Father lives His. Doing this, you become perfected in (and justified by) the Gospel.
Hopefully, this helps you understand my second disclaimer. Does our Heavenly Father choose not to love disobedient children? Of course He loves them—even when they make choices that push Him, His Son, and the Holy Ghost out of their lives.
Recompense to no man evil for evil. Provide things honest in the sight of all men. If it be possible, as much as lieth in you, live peaceably with all men. (Romans 12:17-21(17-18), take a moment to read the entire passage.)
No one is expected to live with a burden they cannot bear. Paul taught:
There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it. (1 Corinthians 10:13)
In summary, for some, the path out is to let God judge the abuser and sever the relationship—but for all, if someone has made a responsible choice based on his/her love for their parent weighted against their right to have (Christian) joy in their lives, then the parent has been honored and the the victim-child is justified before the Lord.
Finally, I might be wrong, but you appear to be pressing me for a single, definitive answer. If that's the case, you're not going to get one. There are people in the world with strong enough personalities that they can and should preserve the relationship in the hope of bringing the abusive parent to Christ (or assuring their judgement if the parent doesn't)—just as there are some who are not strong enough and must sever the relationship or risk cursing God and losing their own salvation. Where any specific victim-child sits in that spectrum is beyond the scope of this site.
1 LDS bishops and branch presidents are pastors. Bishops preside over the common congregational units of the church. Branch presidents over small, often rural congregations. I was a branch president.
2 This kind of cross-interpretation is permissible based on 1 Nephi 19:23, "For I did liken all scriptures unto us, that it might be for our profit and learning."
3 I respect the irony that this verse, if applied without likening the scriptures (see footnote #2) means that the victim-child would be cursed by God for acting as directed. I'm willing to go out on a limb and suggest that interpreting the Lord's commandments as "the blessings of the Lord will be removed and remain removed even if it affects later generations until repentance occurs" is fair. My colleagues will correct me if I'm wrong.
4 LDS Theology is strongly influenced by the idea of councils and counseling (e.g., compare Alma 37:37 with Doctrine & Covenants 1:38), both ecclesiastical and professional. However, a discussion about councils and counseling in the LDS Church is beyond the scope of this answer.