If any node can challenge (not only miners), then there would be a low risk of such a collusion. Or am I understanding wrong?
In that case: which one of the 12000 challenging nodes gets the block reward?
the case censorship could be a problem eventually
If we're going to have large protocol changes anyway, wouldn't it make more sense to make transactions private by defualt (like Monero)? You can't censor what you can't see.