The topic really carries over to the discussion of altcoins. Apparently, for Bitcoin, any change in the algorithm is unacceptable.
Says who? Bitcoin is not patented by anybody and no one is entitled to speak on behalf of it.
If some node intentionally does not sign the message, then the miner includes in the block any other UNI with a minimum value.
And it is the problem, unintentional forks happen too much because you have not addressed the availability nightmare, There should be a consensus on what miners should include in their blocks before they start mining. I'd suggest a pre-pow phase. e.g. instead of including UNIs in the blocks let's have a PoS-only empty block each round which confirms the PoW blocks and is confirmed later by another PoW block and so on.