While you definately need rollback I wonder if it would have been easier, if a player got into a conflict position, to just dump their state and grab it a fresh copy from the server?

As long as you'd sent all their moves off to the server, they should see all the moves they've done which were valid (hmm.. there could be a race condition there). I wonder how often it's worth doing the more complex unwinding described? I may well be missing some complex case (which I imagine is often the case with rollback!)