Some monster abilities (Split, Undying etc.) allow to replace defeated monsters ignoring group limits. If there are not enough minion monsters left in stock (such as in a 4-player game where all available figures usually are on the map at the start of a new game) and only one of the master monster has been defeated so far, which figures may I use for the replacement? If none, then the ability has been wasted?

Buy a second copy of the game; I'm sure FFG won't mind. You'll also get an extra set of those much desired dice.

Just use a placeholder model. It doesnt matter. If you buy the conversion kit you NEED to use all models available. Its not something uncommon, just make sure everyone knows what the model represents, and you are good to go.