You guys are really trying to find a utopic fits-all solution.
the problem is: single player mode.
the problem is NOT: a perfect sentient AI
so, let's get some basic design ideas, shal we?
1. maps should have "hot spots" for building things. like, mine data from every multiplayer game, and chart the most comon places for sentry guns. mark that as a hot spot for bots to build.
2. mark hot spots for ambushs. like, places where the low aliens would go to and wait some minutes on the ceiling waiting for the player. no need for an AI. just mark previously on the map.
3. then, all the hard part will be dog-fighting. no need for a very nice team strategy, since no one ever saw that in multiplayer anyway

3.a. dog fighting can be all about hit as hard as your weapon/jaw/paws can, until you have like 15% life, then run for your life to recover.
3.b. recal from other bots engines that the personalities revolve around things like "thresold for runing for your life", "rate of fire vs aim"... not to hard to do programaticaly.