re Costs: That's actually how I found out that spells.txt wasn't affecting the real costs - I left Basic costs alone, the Expert and Master levels were 75% and 50% the Basic costs, respectively. The only spells whose cost-at-cast were actually changed were the ones that change to begin with (Spirit Arrow, Fire Arrow, Cold Beam, Static Whatever, and Magic Arrow). The spell description reflected the expected values, but on-cast was still full cost. Which I guess is a good thing - can you imagine being able to throw around Dark Containments at only 100 per?
re Mass Curse: It's hardcoded? That's a bummer. Still, I've wanted to learn asm for a while. The main issue with curse is that it applies a chance of failure when casting spells. Which makes it next to useless against most enemies (and the ones where it would be worth using against are virtually immune to it anyway!) Since monsters either cast one spell or else do an 'attack' it's meaningless. Because of the cost, feeblemind is better (since that actually shuts down the spell instead of giving it a chance to fail - the only thing MC has is group effect).
re In/Outdoors: Yep, they're different file formats. Probably some header shenanigans that gets checked. I'll poke around the map files some more. As for skydiving, fall damage is still applied indoors, it's just once it's relevant you've always got Day of Protection running anyway.
re Paralyze: I'm not sure which would be cheesier - changing the spell's resist type, or removing the immunity. Lore-wise no magic at all should work on Gold Dragons, but you can still shrapmetal them to death (not that you'd usually WANT to - it works best at point blank).