Ad costs: I thought as much. It's hard to miss
Ad mass curse: Yes, it's a useless spell (like many others ). MM6 is nearly as old as me . Those days you wouldn't think of being friendly to modders and you would just hardcode everything. (And I don't think I ever found anything that would classify as a "script" in the LODs.) Which is sad because a lot of other spells would need to be fixed (like mass fear, turning undead, mass distortion &c., you know what I mean) // And I wouldn't ever bother with feeblemind either. As you correctly say, the monsters have only a chance to fire one spell.
Ad in/outdoors: Yes, it's quite possible that the game recognizes it just from the map file format. As for falling, there was a discussion on the board some years ago. People tested it and concluded that the fall damage is not applied indoors. Try searching for "Skydiving in VARN".
Ad Paralyze: Yes, you can shrapmetal them to death, because the "magic immunity" doesn't mean immunity to all spells. It's just an immunity against the "magic damage type". The direct damage spells just throw a missile that has another damage type like physical, fire, poison &c.