Dude... final mission is not even a brainy card. It’s a crazy card. The only character that can play both is the professor.
You’re still talking about one deck. And one card, in one deck. And yes, in your “e.g.” Briar Rose STILL works. Final Mission itself is a 2 for 1 (for you... two cards to destroy your one) and requires leaving up brains (if you recall, my suggestion was to play Rose in response to a Monument that hasn’t been fused yet).
The only situation in which .... presumably using a non-Monument zombie to Final Mission your Rose is really a valuable play for the zombie side in is if your opponent is willing to give up that massive card advantage b/c (s)he is already holding on to a low-cost beater that grows through death. Does that sound familiar? And as far as I understand, Valkyrie and Monument are usually played in the same phase.... so your opponent with the Monument in play is probably not running a Valk deck... so your example does not make sense.
Monument + Deadly is not possible with the professor; this is an entirely different deck/situation. And here, if we want your plant dead, we have to play both zombies on the same turn, turning Monument into a functional Eyespore (except plants can still play in the lane before combat happens, unlike when Eyespore is played on us).
I know you run multiple Eyespore in your deck (assuming your in game name is the same as your name here - I’ve played you), so presumably you think it’s a fair card/fairly costed. Q.E.D. so is Monument used for the same purpose at the same cost.
Playing around a fusion card in play is a guessing game, but it’s the same for the zombie side (when should you oppose a pumpkin shell? what power zombie should you risk putting there? who knows!), and it isn’t much different from deciding how to play around a gravestone. I say if you’re the sort of player who opposes gravestones, oppose an in-play monument with a throw-away high toughness plant. If you’re not, you can ignore it in the early game (i.e. Valk costs 3, but he does come out turn 3... keep track of those zombie deaths).