I'm still relatively new, so take what I say with a grain of salt...
1. Assgrens aren't a great substitute for Grens, though they do work good in conjunction with each other. Can't speak much for Ostruppen.
2. While they definitely seem to be the weakest when compared directly to USF Riflemen, I very rarely use them in a pure 1v1 fight. There is always an HMG, Mortar, or Sniper being used in conjunction. I tend to use my Grens as a lure sometimes. One model dies quick, the other three seem like an easy kill.
3. A forward retreat point for Ostheer would be great, but I'm not entirely sure what it will fix. Grens still have the same combat abilities, they can just get back to it quicker.
Anyways, correct me if I'm terribly wrong but isn't the point of the early Ostheer game to utilize all your different infantry in some sort of a triangular support system to control the map until you can get a decent vehicle out, or simply lock down the map for an early game win? I never envisioned that my Gren squads could compete with most of the other infantry out there. To me, they are meant to be used in conjunction which is where the true effectiveness lies.
You don't need grenadiers to play this support role,Osttruppen won't wipe and dirt cheap reinforce.Alternatively pzgrens will protect your mgs much beter while having near same long range dps as LMG gren,they have vet bonuses that are competitive with rifle/IS,they aren't a muni sink either(which u can invest in tellers) and individual models die slower due to rec.accuracy.Grenadiers are a MP hole with frequent 30 mp reinforce and lategame will be wiped for sure.They also are useless assault troops as they can't fire on move,ur mgs may save u on defense,but on offense u need core infantry-especially for meeting engagements.
Rifle grenade is a joke and another munition sink.Faust has been indirectly nerfed.
Grenadiers overall are no longer a must have for wehrmacht.Infact their pathetic weakness will often cause you losses if you use them.