As it stands the USF vs OKW match up is quite largely unbalanced.
I agree with Corsin about US officers being given heavy weapons when they come onto the field is over powered. If the US infantry wasn't already super strong with instant suppression from the Rear Ech troops and rifleman being good at all ranges then maybe it would be fair. However as it stands this is not the case.
The USF can also field the M20 or the AA-HT track around the 5-6 minute mark and they are incredibly strong for how cheap they are. The fact that the M20 crew has a bazooka just makes the unit even more ridiculous. In my opinion it's armor should be the same as an Ostheer 222 scout car even with the skirts. As for the AA-HT it absolutely tears apart all light vehicles and infantry with no set up time at all compared to the OKW Flak HT which comes around the same time. Even the puma has to be wise around it as it does so much damage. Maybe a solution to this problem is add a set up time to the AA-HT and or make it weaker or perhaps slower.
Another suggestion to fix the balance could be putting the panzer 2 into tier 3 instead of tier 4. It seems incredibly strange to have such an average T-70 like tank in such a late tier where only the elite units should be.
Some serious adjustments need to be made to the flak-ht at any rate. It's setup time just makes it useless and will end up being the reason it gets destroyed. As soon as any armor or at gun gets eyes on it it's dead. This is because it will always get two 'shotted' as it can never pack up and run away in time. It's smoke ability will not do it any good against decent players either as they know how to use attack ground.
While I agree with the issues you listed, I think your approach to fixing / balancing them is not good (On a personal sidenote, I think OKW vs USF is probably still easier to win than OH vs USF).
The officers start with weapon upgrades, but are also more expensive than a normal riflemen squad. More popcap, more value, more reinforce cost.
M20 and 221/222 are pretty in line. With 11 / 5.5 armor and 240 HP for the M20 and 9 / 4.5 armor and 200 HP for the 221. Their guns are similar, though the M20 can tackle light vehicles better at short range due to higher penetration. The price 340 / 20 for the M20 and 210 / 15 for the 221 in my eyes makes those differences acceptable. My only current problem with the unit is the sideskirts upgrade, as it improves survivability a bit too much. The bazooka crew is good, but also risky - losing a M20 means it can drop a bazooka.
AA-HT is definitely a problem. I don't think their timing is a problem, as you can get the Oberkommando FlaK HT shortly earlier or a puma shortly after. They just vastly overperform for their cost. Suppression on HMGs as well as on the main gun, better DPS than an Ostwind and a good enough penetration to fight off even light tanks. I'd primarily nerf that unit, as I think it is one of the main causes why the USF are so strong right now.
Pz II in any other Tier buy T4 is a bad idea. Putting it in the medic truck would make this building far more appealing, putting it into the repair truck would create a really bad situation for USF or T1 Soviets. Right now a ~4 to 4:30 FlaK HT is no problem for Oberkommando and people hated it until they found out how easily the M15 AA HT counters it. Now imagine a 5 minute Panzer II that can fire on the move, kite perfectly and has a place where it can afterwards just drive to get repaired. Mediocre micro requirement, low chance of losing the unit (only Vet1 riflemen get rifle AT-nade, no AT on the field, can already beat the M15 AA HT right now, even more assuming that gets nerfed) and no need to bind another squad to repair. I think the PII has some timing issues at T4, but that's literally the only tier you can put it in :/
Regarding Riflemen and Rear Echelon. The latter definitely need a nerf / rework of their volley fire, while the first are really hard to balance I guess. They need to be able to deal some long range damage to fight Sturmpios, but not too much because Gren / Volksgren should outperform them on range in my opinion. You cannot buff their short range damage too much, as Sturmpios should win against them at that range. I think most nerfs to the Riflemen should be indirect nerfs, even if they do not fit the weapon profiles so much. Decrease Sturmpio mid/long damage, but increase short. Slightly decrease rifle long range damage. Then if needed, more nerfs. I'd first go with a worse moving modifier making them deal less damage on charges.
Besides those points to Hans I have to say I almost completely disagree with OP. 3/4 of the posted things are outright wrong or problematic. Since OP did not put the time into actually researching a lot of the prices or designs of the unit I don't see a reason to argue.