Here I answered your questions regarding the minicomics:
*When was the Sorceress explained who she was? Were we meant to assume that she was the Spirit of Greyskull from the earlier comics?
They never explained her origin in the minicomics. The only things mentioned about her past was that she used to be the Green Goddess in the He-man and the Power Sword who gave the hero the technological artifacts to battle evil and later they retconned her character two other times in the other minicomics. The Spirit of Grayskull and her two whole different beings. See the Sorceress as the guardian/keeper of Grayskull and the Spirit as the power/essence of that magical place.
*When was it explained how He-Man came to be owner of the Sword of Power?
They never explained that in the minicomics, only in the Filmation cartoon show.
*Where did Adam come from? He was suddenly He-man/Oo-Lar from a tribe to being Adam, the King & Queens son.
The writers of the third wave of minicomics retconned without any explanation the character of the jungle man to become Adam, the Prince of Eternia.
*When were Evil-Lyn & Buzz-Off introduced, I know which was their first appearances, but again, were we just expected to know who they were?
You answered your own question.
The minicomics aren't story continuity friendly. Mattel back in the day keep switching writers and with them the stories. I hope to have answered your questions.