Need to vent, plus general thoughts on grand strategy.

I had another thought, too - when you are rushing, the aim is to HAVE ALL THE ENEMY CITIES AND UNITS DESTROYED. So having your allies steal the occasional city is not an issue, unless you are rank-grinding or you want a level 7 city to spawn or spam triple grenadiers etc. IF speed-running, who cares? Let them take the odd Level 6 town; if grinding, that's when you want to keep them out (starve them or turn them neutral/hostile).

Long and the short - if you want to win, you have to be unafraid to stab your allies in the back.