What are the teachings or biblical understanding, why God assigned Lucifer to guard the Holy Mountain?
God in your citation on the Book of Ezekiel has chosen an angel to guard the Holy Mountain, and not just an ordinary angel but a "cherub" and not only an ordinary cherub but the most beautiful cherub. Let's look at the wordings;
"...Thus saith the Lord God; Thou sealest up the sum, full of wisdom, and perfect in beauty.
Thou hast been in Eden the garden of God; every precious stone was thy covering, the sardius, topaz, and the diamond, the beryl, the onyx, and the jasper, the sapphire, the emerald, and the carbuncle, and gold: the workmanship of thy tabrets and of thy pipes was prepared in thee in the day that thou wast created. (Ezekiel28:12-13)
What would the "beautiful anointed cherub" would guard in the Holy Mountain?
St.Paul in his writings had given as a clue, because he said, God has concealed something from the angels. What is the one thing that God concealed thru the angels?
God’s Wisdom Revealed by the Spirit
6 We do, however, speak a message of wisdom among the mature, but not the wisdom of this age or of the rulers of this age, who are coming to nothing. 7 No, we declare God’s wisdom, a mystery that has been hidden and that God destined for our glory before time began. 8 None of the rulers of this age understood it, for if they had, they would not have crucified the Lord of glory. 9 However, as it is written:
“What no eye has seen,
what no ear has heard,
and what no human mind has conceived”[b]—
the things God has prepared for those who love him—
10 these are the things God has revealed to us by his Spirit.
The Spirit searches all things, even the deep things of God. 11 For who knows a person’s thoughts except their own spirit within them? In the same way no one knows the thoughts of God except the Spirit of God. 12 What we have received is not the spirit of the world, but the Spirit who is from God, so that we may understand what God has freely given us. 13 This is what we speak, not in words taught us by human wisdom but in words taught by the Spirit, explaining spiritual realities with Spirit-taught words.[c] 14 The person without the Spirit does not accept the things that come from the Spirit of God but considers them foolishness, and cannot understand them because they are discerned only through the Spirit. 15 The person with the Spirit makes judgments about all things, but such a person is not subject to merely human judgments, 16 for,
“Who has known the mind of the Lord
so as to instruct him?”[d]
But we have the mind of Christ.
The angels are not gifted with the Spirit of God unlike man, the angels are ministering spirits and therefore was not given the Holy Spirit to dwell in them like God has destined man to become the Temple of God because the Holy Spirit will dwell in the hearts of men.
In the Heavenly Realm, God has chosen a beautiful cherub and "anointed" him to cover with his wings the "Hidden Wisdom" of God in the Holy Mountain, why? It's not yet the "fullness of time" to reveal the "Wisdom of God" in the Salvation of Man to all the angels.
How important is the Wisdom of God, I mean how can we compare it to all the treasures in Heaven and on Earth? King Solomon described the infinite value of the Wisdom of God;
NATURE AND INCOMPARABLE DIGNITY OF WISDOM
For in her is a spirit
intelligent, holy, unique,
Manifold, subtle, agile,
clear, unstained, certain,
Never harmful, loving the good, keen,
unhampered, beneficent, kindly,
Firm, secure, tranquil,
And pervading all spirits,
though they be intelligent, pure and very subtle.
For Wisdom is mobile beyond all motion,
and she penetrates and pervades all things by reason of her purity.l
For she is a breath of the might of God
and a pure emanation of the glory of the Almighty;
therefore nothing defiled can enter into her.
For she is the reflection of eternal light,
the spotless mirror of the power of God,
the image of his goodness.
Although she is one, she can do all things,
and she renews everything while herself perduring;
Passing into holy souls from age to age,
she produces friends of God and prophets.
For God loves nothing so much as the one who dwells with Wisdom.
For she is fairer than the sun
and surpasses every constellation of the stars.
Compared to light, she is found more radiant;
though night supplants light,
wickedness does not prevail over Wisdom.
St.Augustine and the Church Fathers said that the angels in the Heavenly Realm was in a probation to exercise their gift of "freewill and superb intellect" to follow the Will of God and worship him. The angels are not yet tested for their fidelity, so Lucifer including the fallen angels were found to disobey God's Will because of the sin of pride. They want to follow their "own will" and in unison they shouted the first evil word "Non Serviam", and they were all cast out.
Nothing defile shall enter the Wisdom of God in the Holy Mountain, and so Lucifer the "beautiful anointed cherub" can no longer fit to guard the Holy Mountain.
What's in the Holy Mountain, King Solomon when he pleaded God had said;
"give me the wisdom that sits by your throne, and do not reject me from among your servants. (Wisdom9:4)
The Hidden Wisdom of God represents the Ark of the Covenant, it contains the "Mercy Seat" and the "bread or manna" and the "rod of Aaron". St. Paul described the Hidden Wisdom of God as the Cross of Christ as it is His Divine Plan for the Salvation of Man.
God Willed that His Wisdom be concealed fisrt to all the angels and so had "anointed" a cherub to cover it, but in the Old Testament, God had revealed His Divine Plan prophetically thru the creation of the Ark of the Covenant. In the New Testament the Divine plan of God, His Wisdom was now revealed thru the chosen Woman who became the First Tabernacle or the New Ark of the Covenant.
Further expalnation on the New Ark can be seen here on this link;
Why are the offenses against Blessed Virgin Mary are called blasphemies instead of sin?