I’ll take a crack at these…
One of the principles that governs the LDS Church is “Common Consent”. D&C 26:2 states "And all things shall be done by common consent in the church, by much prayer and faith, for all things you shall receive by faith. Amen.
Regarding “Common Consent” Elder Bruce R, McConkie wrote “administrative affairs of the Church are handled in accordance with the law of common consent. This law is that in God’s earthly kingdom, the King counsels what should be done, but then he allows his subjects to accept or reject his proposals. Unless the principle of free agency is operated in righteousness men do not progress to ultimate salvation in the heavenly kingdom hereafter.”.
Both local and general officers are presented to the pertinent congregation for acceptance. A similar process happens with LDS scripture. The last time this occurred that I’m aware of was in 1976 when the revelations that later became D&C section 137 & 138 were canonized as LDS scripture. See https://www.lds.org/general-conference/1976/04/the-sustaining-of-church-officers?lang=eng at the 6:48 mark to watch the proposal for inclusion as scripture and the members accepting the proposal.
The Adam-God doctrine was never presented to the Church for acceptance. Prophets after Brigham Young did not teach this doctrine. So, when you asked when Mormons stopped believing it, is was never presented to the LDS Church membership for formal acceptance.
Polygamy (or “plural marriage” as LDS refer to the practice) is NOT necessary for exaltation/Eternal life in LDS doctrine. The Book of Mormon states in Jacob 2:26-30:
26 Wherefore, I the Lord God will not suffer that this people shall do like unto them of old.
27 Wherefore, my brethren, hear me, and hearken to the word of the Lord: For there shall not any man among you have save it be one wife; and concubines he shall have none;
28 For I, the Lord God, delight in the chastity of women. And whoredoms are an abomination before me; thus saith the Lord of Hosts.
29 Wherefore, this people shall keep my commandments, saith the Lord of Hosts, or cursed be the land for their sakes.
30 For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things.
In a nutshell, the Lord forbids polygamy, except (per verse 30) in certain times when He commands the practice. Monogamy is the rule, and plural marriage is the exception.
PART 1 OF RESPONSE, MORE TO FOLLOW…