Some things I would like to point out that you might have missed or should be considered:
A - In regard to the AI: Did anyone else noticed that the enemy mech/vehicle diagrams (when selected) showed pilot tag designations, like the ones you get when you start specializing your pilots - Gunner, Tactician, Defender; etc. ...
Maybe that has something to do with why the AI does not always makes optimal choices?
Especially when the pilots get procedural generated and randomly assigned to mechs/vehicles...
B - In regard to salvage: I think its fair to say that Cohh got lucky when he managed to salvage three mechs on that one mission, even so he negotiated for it.
But after the mission he still had to put one of those mechs into storage, plus two other mechs from his active mechbay (cause he still had only one)!
Now, if I remember correctly, the devs said, that mechs you put into storage get stripped of weapons and armour.
(In the PDX/HBS stream on the mercenary campaign with Kiva and Mitch at 00:40:43)
Which would mean, to get them battle ready again, you would have to spend time! ...and money?
And upgrading to additional mechbays is a financial decision as is having a large number of battle ready mechs to choose from, since you have to pay upkeep cost, even if its the same amount for any mech.
Edit: Also, if you managed to leg a mech, head cap it, or kill the pilot before you destroyed any of its components, than why wouldn't you be able to salvage a complete mech?
I haven't checked back on Cohh's streams to see, if destroyed any components on the mechs he salvaged before he took them out.