Apple did not bend the knee in the UK, it forced the hand of the UK to reveal it's goals. Obviously we will see, I don't have much hope either. As for Signal, I hope they pull out as it will get media coverage somewhat on this issue.
I have understood that Whatsapp is not a terribly profitable product for Meta, so it is possible that it would just be withdrawn from Europe, instead of making expensive and controversial modifications.
iMessage is not really a thing in Europe. Apple phones are simply not popular enough here for it to be an useful feature. I guess Apple would just disable it for European users.