If you started out even and now have 5x their score, they aren't going to catch up.
It may be suicide, but they sure as hell aren't going to have a chance without ganging up on you...
As for the dominoe surrender affect, if it was changed so you needed 50x their score instead of 10x, everything would be fine. Either that, or a minimum number of turns/years before they would consider a surrender.
The current problem is that they'll give in within the first few turns, they start slow, and that they can't fight tactical combat worth a damn. If you fixed these, the surrender issue becomes a non-issue, because you wouldn't be able to take out 15 AI with a few escorts in 20 turns then.