Kefka final battle and Statue of the Gods 3 tier Question

Yeah, full swapping is always going to crash no matter what because you'd need four full parties in order to avoid an empty last battle, and the ordering function before the fight doesn't allow that even if you force form four parties before you invoke it.

You might be able to set a RAM value to how many characters you have available... with an event before the fight begins, or in the party ordering screen, or even during the special handling of the first phase of the fight (the check for the formation background). You'd then decrement this RAM value by 1 each time you swap out a party member, and refrain from further decrementation or swapping once it reaches a certain threshold (4? 1?). The trick is calculating how many party members you have in the first place... not sure where to start with something like that.