I -think- I may have solved the problem with the Stuka's kamikaze diving, it appears to be caused by the AI input to select the rear gunner position (in objects.ai) being set to PIMenuSelect2 (which should be the key to switch between 50kg/250kg bombs), rather than PIPositionSelect2. I added the bomb select key (missing from the default files). This didn't cause any problem in default FH2.25 as there was no bomb select key in weapons.ai, but I added one (without spotting the error in objects.ai) so presumably when the bot pilot got within range of the enemy and attempted to select a bomb type, he instead started switching between the pilot and gunner seat, causing the plane to crash.
Fixing this error makes the Stuka much less prone to crash into the landscape, but it still flies very sluggishly, doesn't attack anything, and usually ends up getting killed by flying out of the playable area. The AI control input parameters for the Stuka are also set to be much less sensitive than other aircraft (including the Ju52); I suspect this is the cause of the latter behaviour, so will change it and see what happens. Still no guarantees I'll be able to get bombs working, though...
I haven't really looked into the fire rate issue with semi-auto rifles yet, but I suspect it's because these guns don't actually have a fire rate defined in their .tweak files, they fire as fast as the player can click the mouse button. Since bots don't need to use a mouse, they can fire them a lot faster than a human player
I've also done some more tweaking to the tank gun + MG files, and have made a couple of changes that seem to make tank vs. infantry combat work better; firstly, I gave AP shells a strength of 0 vs. infantry for all tanks that have HE shells, secondly I gave coax MGs a much higher strength (15) and reduced their range to 150. Within this range tanks will prefer to use MGs rather than shells against infantry, beyond it they'll fire HE shells - at least in theory. I think part of the reason for the erratic behaviour of tanks was that HE shells and MGs had a similar strength vs. infantry, the bots couldn't decide which to use!
I've also reduced the minimum range of AP shells to 1 and HE shells to 5.