Here's a question;
Could one define a light machine gun (vs automatic rifle) by the crew assigned? It's all a matter of semantics, really, but since they were both used as squad automatics, this gets into tactical doctrine, and each nation used the doctrine and terms they preferred.
Was there an "assistant BAR gunner"? I know other guys in the squad would hump ammo, but was there a designated guy to feed the BAR (other than the BAR gunner?)
Did the Japanese use an assistant gunner for their magazine fed LMGs? Several other nations did, and I know they Japanese did for the strip fed guns.
SO might one not make the dividing line at automatic rifle (crew:1) and LMG (crew:2)?
As to liquid cooled engines in aircraft, they are more vulnerable (one more thing that can go wrong/be damaged). However, I do agree that their vulnerability was much over blown. That being said, there are documented cases of a single bullet bringing down a fighter due to a hit in the cooling system. Rare, but it did happen. When the stars line up the right way, it happens.
IIRC, Combat cars were "tanks" (full tracked, w/turret(s)), but didn't have cannon, only medium (.30cal) and heavy (.50cal) machineguns. Yes, some people got combat cars, because they weren't authorized tanks, and congress, being what they were, would fund "combat cars", because tanks were too expensive....
All else being equal (and it almost never is) bigger bullets tend to work better.