Only one I can really answer is the indirect death thing; in general, any enemy you've had contact with that dies to means beyond yourself will still give you XP. In the case of confusion death via drowning or suicide, etc, you definitely get the XP if you inflicted the confusion. If they just off themselves in your sight, I'm not so sure.
Re: Poison and Draining,
Poison Arrow can be used throughout the game, but draining ultimately ends up pretty 'meh' and the rest of the poison spells situational, i.e. only against non-resistant critters. Mystic blast is pretty solid throughout if you can keep space between you and the target and is pretty easy to cast regardless as to if your conjuration is high or not, so it's an option. I'd imagine your best bet as a necromancer would be hunt down necromutation and haunt, then spin the two together to wreck everything in sight. Otherwise check the wiki guides, which do alright for such information.
Also check out
the knowledge bot, which is tremendously helpful.
EDIT: And listen to Feelgood, he's definitely more experienced with crawl than most of us here