Profile of nee

Never heard of G43s having better performance against units in cover, just better performance in general.

Yes. IR StGs improve firepower against cover, much like flamethrowers. If you have the resources just have two Obers with one using each.

I have personally not found the ability very useful in terms of damage. Often you risk losing a few guys closing the distance, so you're not always going to throw all five grenades. The grenades don't even seem very powerful compared to the single 30 munitions cost. I tried it once against a large blob with about two whole Volks squads, and it just killed one guy, and dinged the rest that were in range. Since it is multiple grenades you cannot anticipate exactly where they go, either.

Honestly I'd rather they get a stun grenade instead, those things use one grenade and stun everything nearby.

TBH I don't see how these grenades fit into an ability called "Infiltration". More like "Run right in guns blazing and blow that shit up!"

Reading the posts the main problem of the Kubelwagen (KW) is that it's poor in comba. But what if it's role was geared more towards utility?
Capture points, scout for allied infantry, and even lay mines or something, as long as it's not engaging other things. It is after all a lightly armed car, even lighter armed than 222 scout cars, and even those vehicles weren't meant for combat, more for reconnaissance. Even it's ability, Detection, leans towards this role. The current problem of course is that it has no other role but fighting. Give it capabilities like capturing points and its purpose becomes HUGE, all without having to change its stats and such.

If not a default ability, then perhaps future commanders could incorporate the ability for light vehicles to capture points, including the Kubelwagen. This vastly expands the capabilities of the KW (and others) as a unit throughout the entire game as it can be relegated to capture duty rather than to the few infantry you have on hand. You won't try to bring out a KW to engage that Major and his bazooka-armed friends. You send in your veteran Sturmpioniers...and the KW goes to capture points. The doctrinal ability alone (assuming its for all light vehicles, not merely for the KW) already makes this a very powerful commander, because you are not wasting your few infantry doing things besides shooting up the enemy.

I'm asking because it can influence my opinion....

I haven't check this, is the repair speed of the vehicle's crew the same as engineers ?

I would assume so: the repair buttons for both crews and RETs are the same, whether in vet0 or vet3.
Vet2 is the only one that mentions repairs: "Innovative mechanical techniques improves the squads repair time". Vet1 unlocks the critical repair, and vet3 makes them more durable when repairing, which indirectly affects the repair rate. Rear Echelons also get a repair bonus in vet2, though "improve" in the game's terms is a bit vague; does it increase critical repairs or merely accelerates the time?

I feel like USF vehicles just have this ridiculous independence because they can self-repair on the cheap.

It wouldn't be so bad if it was riskier. Maybe make the vehicles easier to decrew or something. Right now there's not a whole lot of risk involved.

Other factions have similar methods though, namely the Soviet T-70's self-repair, their doctrinal Repair Station as well as Conscript Repair for a number of commanders. Elite Armoured Doctrine for OKW also allows self-repair, but it is an inferior, decrewable version of the T-70's.