1.Yes . Hidden units can be replaced.
2.By ordering them via triggers, yes. but by user control no. But I think its possible via Get local player.
3.No hidden units are not affected, check that by using using a buffed DPS spell on an area and then unhiding the unit the answer is no it wont be affected.
4.You mean selecting them to a player ? Probably not...but I'm not sure on this one...