This sounds like it should work as well, it doesn't matter if you enter an invalid state as long as you're forced to revert to a valid state before you can close the menu. Just be careful about implementing autosaving, new ways to cancel the menus (etc) so the player can't end up stuck with too few monsters that way.
Viewing post in Yal's Monster Collector Engine comments
Yes for sure, I realised as well that there is not a release function in the box menu, at least, not one I can see??? So if that's the case and I implement one, I'll just need make sure that the party + box monsters is greater than 2 before you release a monster 😌. But anyway, big thanks for your help and the engine, has been a little tricky to get my head around some things but its progressing nicely
Glad I could help! I've done my best to make everything as simple and easy-to-understand as possible, but between the big sweeping GML changes since I first created this project butchering some of the organizational aspects, and an RPG being pretty dang complex to begin with, things can get a bit overwhelming.
(Btw, if you haven't already, you might want to check out the old comments as well, I've answered a lot of programming questions for misc common features over the years and if you have a future question it might've already been answered in there)