But you have analyzed these things much more than a casual player like myself could even imagine.
Is that just a nice way of saying you think I have no life?

It's alright, my wife would probably agree with you.
The AI will target the ship with the highest attack per hit point. So...
If you have two ships, one with attack 20 and hp 10, and another with attack 20 and hp 20, it will target the ship with less hp, because it figures that ship has 2 attack per hp. This has the advantage of making your more experienced ships be fired at last when in a fleet of other ships of the same design without experience.
For this reason, however, a mix of offense and defense ships doesn't really work. Your defense is useless on the defensive ships, while all of your offensive ships just get blown away. There are a few solutions to this problem.
- Sacrifce attack ships. Send in one attack ship all by itself with a high attack value to just weaken enemy fleets, before you send in your REAL fleets. You, like the AI, will automatically take out the enemy ships with the highest attack power. This will leave your target much weaker when the big experience boys move in to play mop-up.
- Use fleets of identical ships. I especially like this strategy with huge all "all armor" models, because their offense can be tiny, and you don't want your whole fleet to go *poof* because of the tie rule. If you aren't taking damage anyway, you up the survivability of the whole fleet, and then the whole fleet gains experience for everything you kill. If you are using fleets of identical ships, if after a battle one is weakened to a level that makes you uncomfortable (under half, under 25%, whatever), just remove it from the fleet for the rest of the turn, and continue attacking with your healthy ships. Then, at the end of the turn "fleet up" again, so that the enemy can't pick off your stragglers.
- Mix large and huge hulls. This doesn't work well, but at a point in the midgame is OK. See, large hulls can still pack a lot of defense. Your large hulls are taking the bulk of the damage in this scenario. The Huge hulls I use for this also have a high defense, but still a decent attack. Look at this scenario:
You have a large hull, 80 hp, 40 attack, 100 defense. You fleet it with huge hulls, 120 hp, 55 attack, any defense (or whatever). In this scenario, your large hull has more attack per hp (.5 vs. .46 for your huge), so the large hull will be targeted. But notice your huge hulls actually do more damage.
This setup works well until enemy fleet attack values start to rise above what a large hull can handle... and depending on your tech settings and difficulty level, that can happen really fast. At this point, you are going to need to switch to using all huge hulls.
Even then, this setup isn't great, because if you have already researched huge hulls, you could probably do better right fromt he get go sticking with them.
Anyway, hope that helps.