Answering the ones that I know off the top of my head…
(2) There’s a couple different use_actions available: PKILL_1, PKILL_2, PKILL_3, PKILL_4 and PKILL_L. There’s also corresponding “diseases” (long-term player effects), DI_PKILL1, DI_PKILL2, DI_PKILL3, DI_PKILL_L. There’s no DI_PKILL4 for some reason, there’s no level 5 at all, and all those effects are hardcoded in the game’s source.
(4) Nope, use_action can have at most one entry at present.
(5) Hardcoded :-/. The relevant information isn’t located in a very central place, either…
(6) Other than editing the recipes file, you’ll need to edit mapitemsdef.cpp and recompile the game. I’m gonna add a note on the todo list to move that to data files, though, because it’s low-hanging fruit, and is something that really should be possible.
Overall: it’s currently a little tricky to mod drugs. Drugs are a case where you really should be able to mix and match a bunch of parameters, including tweaking numbers, but at present most of the relevant parameters are hardcoded. That’ll probably change in the future, but I don’t know when exactly.