I think the original questions that started this thread were good ones. I also think good responses have already been given, but I thought another attempt would not be detracting, so here goes:
āHowever, isnāt it enough to trust in the mercy and power of God to save those who may not have understood the fullness of Truth?ā
That seems logical enough, since certainly God is all-powerful and all-loving, and merciful.
But the question boils down to asking what baptism is for anyway.
If baptism is strictly to provide evidence to God that someone has made a covenant with Him, then it could be reasoned that spirits in the spirit world could make a covenant that God would acknowledge without needing baptism to do it.
But if baptism is more than just āevidence of a covenantāāif it really does have a physical significance and a spiritual significance that both our physical body recognizes and our spirit person recognizes, then it makes sense that those spirits in the spirit world would desire that same opportunity for the resurrected body they are someday going to have, as they learn about baptism to wash them clean from the effects of their sins, through the atonement of Jesus Christ.
Baptism is also only a first step. The New Testament talks repeatedly about the ābaptism of fireā or the ābaptism of the Spiritā or the Holy Ghost. See John 3. The two ordinances go together. Baptism by water is not complete without baptism by fire, or the sanctification process that those words mean. Spirits in the spirit world would want that sanctification process just as much as a person on earth who understands how it changes their soul through Christ.
The LDS practice in temples includes baptism for the dead and giving that person the gift of the Holy Ghost, which really unlocks a process of continuation of learning and growing just as in this lifeāfor a person with the Holy Spirit in their life learns and grows in understanding.
The apostle John talks about being purified:
1 John 3:1 Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3 And every man that hath this hope in him purifieth himself, even as he is pure.
This purification process or sanctification process takes place in this life and can also take place in the spirit world. Those spirits there are alive. They think, they learn, they converse with each other. I know this to be true.
Peace to all, this blessed Easter day.