[Endoperez]

1) Very little.
2) Nothing that drastic. Higher Drain makes spellcasting more fatiguing.
3) Yes.
4) Can't say for sure. The exact effect it has on spellcasting manual might be undocumented. I don't remember seeing any details.
5) Certain nations are immune to Drain research penalty, and others have insanely cheap researchers that benefit very well from Magic. For those, the choice is always easy. Some nations don't need magic much, so they can survive with Drain. Niefelheim, perhaps, with access to good infantry and good SCs in Niefel Jarls.