I didn't play, leave alone modified, the game in ages, so this just comes from the top of my head.
Costs: probably a dumb question, but there is a separate cost for Normal, Expert and Master skill levels. Did you take that into the account? If the number in spell description changed, I guess you changed the Normal one. Maybe you tried it with an Expert?
Ad indoors/outdoors: I think that, from the game's point of view, these are fundamentally different. Rendered by different engines, stored in different file formats (I guess) and so on. Even the game rules are different (skydiving outdoors hurts, if you do it indoors, you make it unscathed). That being said, I have no idea about what exactly determines if the map is going to be treated like indoors or outdoors.
Ad Mass Curse: No (at least not without hacking the game executable itself), I'm afraid.
Paralyzing a gold dragon should be impossible. If you look the spell up in spells.txt and have a look in the "Res" (=resistance) column, you'll see "Magic" for Paralysis. And if you look up the gold dragon in monsters.txt, you'll see "Imm" in the magic resistance column .