Specifically about the hack, @rishi556 is right, if your node is hacked and the db gets corrupted, your node will stop matching the rest of the network based on hash mismatch. I will say we were bit lucky here, had we not done the p2p work at the time we probably would not have been able to find all the db modifications as easily.

Anyway, it's important to look at the verified block number across multiple witness nodes to know which block is safe to assume as not modifiable. @cryptomancer recent eth bridge does account for this which will definitely prevent issues due to hacks on a few nodes .

As for other comments about P2P, the ideal state is that the sidechain block production itself is undergoing verification before production, which can be done with a bit more work.