1. Exactly. If there's nothing specifically stating when a card with a hold is added or removed, it's prep phase.
2. No. Matthias' ability activates when he enters battle, so by the time Holy Spirit is put in play that's already come and gone.
3. It depends on if there's a "may". In the case of Wonders Forgotten, there's no "may" so the search is mandatory, meaning that it does count as a search for Music Leaders. ML does say "may" so your opponent gives you a trigger you can choose not to search, and no search ability is used.
4. Music Leader is still able to activate outside of battle phase, so yes he still gets the search.
5. Yes. Placing a weapon on a warrior in territory is not considered playing it.
6. Yes, unity is a constant check.
7. You only get one chance to interrupt the battle. Once you've used your special initiative against a single enhancement, it's gone. That said, some interrupts allow you to play another enhancement (the Horses weapons, Dream, etc) - you still get the play ability and, if you wish, can play another interrupt enhancement from that. What you can't do is play as many enhancements as you want as long as they interrupt - if, for example, you play something that says "interrupt the battle and capture a hero" and choose to capture a hero in their territory, once that resolves you DON'T get to play another interrupt.
8. Yes. Protecting from a character also protects from enhancements used by them.
9. It will still apply to any humans that entered battle while Overtaken was in play, but won't apply to anyone new.
