I can maybe help with the accessory thing. As far as I know, accessories are removed along with Materia with the '17' function in 'Modules - Display Menu' (they're used in Shinra tower to un-equip party members before the Gunners and Rufus fights; these are in the 'direct' group of their respective field screens if you want to look at them).
Ah, thank you. The MENU opcode seems to do all kinds of stuff with your inventory. Well that at least solves that issue. Though that makes me wonder... what if you've filled your materia inventory and then you're forced to unequip them? I might have to test that.
Are you trying to get rid of KOTR for the boss-rush mod? I thought a potential work-around might have been to remove the party's Materia/Accessory by using '17' for each party member in turn (changing 'Short', the field below it, determines which party member it affects). This puts every Materia in the inventory assuming there's space. I'd have thought you could then 'target' KOTR and other banned Materia using Remove Materia under Party & Inventory (it looks like it asks for AP as an identifier as well as the Materia itself) but I just tested it with Fire Materia sitting at 0 AP and it was a no-go, unfortunately.
Yeah, KotR, Mime, Double Cut and the Master X Materia. It also removes Elixirs, Megalixirs, Hero Drinks and Ribbons, though the latter weren't removed if you still had them equipped, I should be able to fix that with the MENU command you posted.
The problem is that both the Remove Materia and Amount of Materia opcodes don't seem to do anything. I tested both ways (either just not letting you into the Hall of Battles or removing the forbidden materia upon entering) both with either 0 or 500k AP and neither worked out. Guess I'll have to rely on the players.