If a team were to retain on a player, wait the 1 year required time, and then reacquire that player. Would team A still have a retention slot being used from the original trade?

That’s an interesting question, that doesn’t have a clear answer in the CBA. Since that situation isn’t mentioned at all, I’d assume that the retained salary is still treated as taking up one of the three spots.

Also if a team retains salary on a player, they don’t need to wait a year to re-acquire the player, as long as the team that is trading the player back doesn’t retain salary as well