You may indeed be overthinking it.
Remember people, Volksgrenadiers only achieve vet quickly because of PANZERSHREKS, and their consistent combat with mechanized units
This means that if you make your Volksgrenadiers become anti-infantry specialists over Anti-Armor specialists. They would then gain vet slower, and any that survive to Vet-5 would indeed have earned it.
Also I find that Vet-5 Volksgrens still aren't quite as durable as Shocktroops, just my opinion though I guess.
I'm not sure about the exact calculations involved with armour vs received accuracy so I can't comment with certainty on Volks vs Shocks. I suspect, however, the the difference in overall durability isn't particularly large between vet-5 volks and vet-3 shocks.
You are correct in a sense about how Volks gain vet rapidly thanks to schreks. That being said, it isn't exactly difficult to get Cons/Grens/Rifles to vet 3 as it is and Volks are the cheapest of the bunch. Add in an anti-infantry package and they should pick up veterancy at light speed - same as always.
I will say again that, if this were to be introduced, I don't see why anyone would bother with Obers again. By the time they become available you would already have multiple vet 3+ volks with automatic weapons that are not only more effective than unvetted (and probably vetted as well, given Volks' arguably superior vet bonuses) Obers, but are also vastly more durable and cheaper to reinforce.