1] WW2 infantry companies had far more MGs available than they get in ASL. 2-3 times as many. Every US PIR squad would have an M1919A4 .30 cal. There would then be a spare one in each platoon [I think] and a few more in a weapons platoon. They actually found they had too many and needed more riflemen in the bocage fighting.
A small correction, I believe until late war (late '44? or '45) the squad had 2 M1919A6 (with stock and bipod). The late organisation had 1 M1919A6 and 1 BAR. That was all in theory. With the possible exception of ANZAC troops the US troops, especially Airborne, had the greatest habit of 'acquiring' extra kit. Then as you pointed out there were MG in the weapons platoon. In substance you are dead right about the profusion of MMG/HMG available to US troops and the understating of that in ASL.
I agree that for trucks the .50" or .30" was much more an AA and "just in case" weapon.
For Jeeps, especially in recon units, much less so. From reading of some Bulge actions, especially if they were there for some time, the recon units dismounted their MG and 60mm mortars and placed them in hard cover. When used for such screening and tripwire purposes they needed every bit of firepower to make up for their lack of bodies.
Halftracks are yet another question. For a mobile action likely at least 1 would have been left on the vehicle, assuming that the unit was intended to continue on further. In a defensive action the h/t would be to the rear with at least a .30" dismounted and integrated into the defensive line. Some would have been left with the h/t as a self defence measure, likely the .50". Then there is also, yet again, the question of how well does ASL represent the number MGs. While the M3 started with just a central pintle MG, the M3A1 regularly had 3, not just 2, MG, 1 50 and 2 30. As far as I can figure out many if not most M3 were upgraded to M3A1 or equivalent with extra stowage and often the MG pulpit. Then there are the usual unofficial additions. Due to having a h/t to carry, ammo would have been much less of a problem. If ASL was accurate then it would be pretty crazy with many more HMG/MMG than squads.
In summary for a h/t borne attack, dismounting one or more does not seem ahistorical. For Jeeps or trucks, unlikely, except for recon, if your Jeep or truck is in or near the front line then likely something has gone wrong, they are not AFV. For a defence the vehicles would not usually be on the board but their MG mostly would. A last ditch battalion or higher reserve for gap plugging would use whatever trucks, Jeeps and MG available and taxi them to the critical point then dismount.