I will admit it didn't occur to me that the gunner could swivel the gun forward and sit on the counterweight. Very dangerous, and not the intended use, but doable. The one on the M10 looks like he's standing on the engine bay, which would mean they cannot turn the turret while he's there. The M10's second mount with the M1918 looks like it's right over the main gun, so I doubt you'd be able to use both at the same time.
Mobility is huge in CoH2 due to the scaled down engagement ranges, so that would not be very favourable. The M18 had a turret ring unlike the M10 and M36, but the M18 isn't in the game so that's a moot point.
And the Hetzer had a remote-controlled MG, JPIV gets Predator cloaking... Shermans can take multiple rounds from a Tiger... COH2 is barely about realism, this is about functionality for Allied Tank Destroyers within the game. Stug and Panther get mg's I don't see the issue with .50 upgrade.
They could just add a guy
standing on the back off the vehicle. Or for gameplays' sake just mount the mg on the front of the turret.