1. If consistent with the idea of cascade (the ability "never happened") music leader's search would be cascaded and the cards put back
2. The reveal is an optional instead. If the opponent doesn't have two good cards, they simply can't choose to instead.
3. By the strict wording of the card, it would have to be discarded when placed or not at all because the only "while this remains" clause is tied to the negate alone. If that card was made today it would would have a colon after "place on a Hero" in which make everything after it "while this card remains" and thus you could choose to discard later. It likely is either understood to work as if it was written with a colon or will receive a play-as to function as if it did in the future.
4. 2 from player 2 and 3 from player 3. The ability and the definition of X both specify it is determined for each player individually.
5. I believe yes because player A is not in battle with anyone both player B and C count as his opponent, but given that player B is in battle with player C, player C is player B's sole opponent so I am not 100% sure.
6. By the current rules, cards returning from set-aside go to territory. If it was a normal enhancement set aside, it will fizzle and be discarded. If it was a character, fort, site, etc, it will simply stay in territory. If it is a dominant its ability will immediately activate. I recall that this is planned to be changed in the next REG update so that the card will return to deck.