As long as you run your own node, it doesn't really matter who runs the rest. If a peer relays an invalid block to you, disconnect from that peer and defer to the chain with the most work.

Have you read the paper?

If a block is accepted by a majority of nodes and miners it isn't invalid.

Your own personal node is free to implement different logic, but it will be unlikely to interact in a functional way with the rest of the network.

The full node operators are typically holding Bitcoin, and I just don't see anyone willingly accepting a reduction in the value of their assets. Any such block would be viewed as an attack on their sovereignty.

There is a way to know since segwit was activated using a user activated soft fork despite not having a majority of miner support

Node control has worked in both theory and practice