[Edi]

1) You need a roughly equally strong dominion and lots of temples plus preaching to stop and reverse that trend and it'll be hard.

2) You can try. AFAK, it has not been tested, but I think they can' be used.

3) It sneds the whole world to cold scale 3, reduces income and generally wreaks havoc.

4) Every 12 game turns is one year. When a unit reaches its old age value, it gets the old age icon. So all warlocks recruited during one year will turn old on the same year regardless of the turn they were recruited on. Death scale increases your chances of getting afflictions due to old age (checked at the end of the year, i.e Late Winter).

5) It's a more powerful version of arch devil but has blood magic instead of fire magic. They each have unique specialties. Yes, it's worth it, in my opinion anyway.