We'll know who produced the block, which would be the most important issue if a block producer was actually trying to break the protocol by allowing a double spend. And, yes, if 3/4 of the block producers then signed off on such a block, you can assume governance is gone.

Further, note that any node (not just block producing ones) CAN keep a record of all those votes, and they would serve as proof of who voted on a bad block (because they would have valid signatures).