As Mason Wheeler mentioned, LDS members consider the answers to these questions sacred, and thus you're probably not going to find much "official" information on this subject. However, there is one quote that comes to mind, and my responses will be in light of this:
Let me give you the definition in brief. Your endowment is, to receive all those ordinances in the House of the Lord, which are necessary for you, after you have departed this life, to enable you to walk back to the presence of the Father, passing the angels who stand as sentinels, being enabled to give them the key words, the signs and tokens, pertaining to the Holy Priesthood, and gain your eternal exaltation in spite of earth and hell.
(Brigham Young, 6 Apr 1853, Journal of Discourses vol. 2, page 31)
According to Brigham Young, these special "key words, the signs and tokens" are used "after you have departed this life". Since the apparent goal is to "gain your eternal exaltation", it seems unlikely that there would be "different levels for different handshakes", since the idea of "eternal exaltation" sounds pretty binary: you either get it or you don't.
If someone who knew the "key words, the signs and tokens" forgets them, it seems unlikely that they would be able to pass the "angels who stand as sentinels". However, I believe in a merciful God, so I also believe that before anyone gets to that point, they'll know everything they need to know to keep going. Thus if you forget something, my hope is that there'll be a refresher course or some sort of way to remember before continuing on.
Do wives have to wait to enter heaven until their husbands die?
This is an interesting question. For starters, LDS doctrine is that "eternal exaltation" can only be gained after the Resurrection, which (generally) hasn't really happened yet (there were those who were resurrected immediately following Jesus Christ, and LDS teachings can point to a handful of cases since then, but the prevailing opinion is that it's largely on hold until the Second Coming). Since you have to be resurrected in order to gain exaltation, the idea of "waiting for your spouse" seems a bit irrelevant, since both would have to wait for the resurrection to happen anyway. Where things get really interesting is on the question of single or divorced people, and the answer I know of to that is "we have no idea, but we believe God is merciful and that all things will be worked out".
If a person later abandons the LDS church, can he still get into heaven, since he already knows the secrets?
That seems unlikely, since the LDS Church teaches that there is more to getting in to Heaven than knowing how to "pass the angels who stand as sentinels". For example, the Church teaches that exaltation can only be granted to those who have received all of the necessary ordinances and who have lived according to the commandments of God. Since some of those commandments have to do with participation in the Church (primarily the Law of Tithing, etc), it would follow that someone who leaves the LDS Church would be ineligible for exaltation, regardless anything they may have received or learned in the past. Of course, the Church also teaches that there is a time granted for everyone to repent and be reconciled before "Judgment Day" when these sorts of things would be an issue.
Of course, since I haven't died and been resurrected yet, take all this with a grain of salt. ;)