Yeah I wondered if instead of maintaining a global ordering of all requests, it wouldn't be better to just require timestamps in requests that will need to be ordered. Then you only need partial orderings, which you can resolve when marging the optimistic state, only among those requests that actually need to be ordered, and requests don't need to block each other.

Edit: although now I think about it, if you go that route you effectively have to do the same timestamp ordering resolution on the server too.. so probably more complicated than just keeping a global ordering!