Requesting a confirmation takes almost the same amount of resources as responding to one, I don't see a DDOS angle other than simple network flooding.
Well, since you don't store historical votes, everyone must be called upon to vote again if a syncing node is presented with a fork, no? You can't just take the opinion of one node at that point, surely?
I still don't see bitcoin as completely trustless, determining the highest block is trustless but you still need to trust where you got your wallet and that it's implementing the protocol correctly.
These points are completely orthogonal to the problem we are discussing.
You also need a way to determine if you're connected to a partition, since there's no upper bound quarum on hashrate there's no trustless way to determine connectedness. With voting you can observe if you've received a quarum and make that determination.
Partition tolerance is the same in both cases; in bitcoin the votes are the blocks.