In biblical times, the Lord commanded some of His people to practice plural marriage…
There is no proof of this. The message given to David by Nathan, “Thus saith the Lord...I gave thee thy master’s house, and thy master’s wives into thy bosom” (2 Samuel 12) could mean the “master’s wives” were given in the same sense that the Lord sent quails to the Israelites, gave them a king, and acquiesced when Martin Harris wanted to borrow the BoM transcript. It could also mean the women were just placed in the care of the king’s household. Every other instance of polygamy in the OT indicates the people acted on their own.
..the Lord…did not give exact instructions on how to obey the commandment.
This is just sad. When God gives a commandment, he prepares a way for it to be accomplished. In the case of polygamy, Joseph sneaked around behind Emma's back and took additional wives long before seeking her approval. The gospel of Christ "was not done in a corner," but polygamy was done in deep, dark corners. He was sealed to women who already had a husband. I think God would have at least clarified that part. Joseph took wives and then allowed them to be kicked out of the house. He vehemently denied he was doing it and ended up dying as a result. I fail to see God's hand in all that.
I could say a lot more, but I'll just say that it's best not to defend the past practice of plural marriage. Someday the church will have to backtrack on this. The more I learn, the uglier it gets, and a lot more will be revealed through the Joseph Smith Papers project.