As someone who plays all 5 factions, your suggestions sound great. Implementation wise will it be too much work? Especially the USF Officer thing, seems kinda complicated.
OKW: the "only" thing which should require
proper testing would be the half tech thing. Besides needing a proper icon i don't see any issue with upgrading the Flak HQ and half tech options.
USF: RET volley changes is basically mixing it up with what they are now testing with Cavalry Rifleman. I would go even further and make RET cheaper to reinforce but make so that they now get an inferior version of 1919/BARs in order to avoid the old spam.
The officer idea. The easiest one to implement is the one similar to withdraw and refit. Right now IIRC there's a similar idea on the SU campaign. It forces a unit to retreat and leave the battlefield. Problem is i don't think there's a way to take into account the number of models in the squad atm with that ability. You could instead reduce the mp to 150mp and make it available despite the amount of models in the squad.
Making units appear with less models has already been done.
Swapping units. This is my line of thought.
Any reasonable swap of units would probably make the LT the defacto choice. Making the Cpt get the core of the LT units is a bit more balanced. If that's it's too much, i would test how a 4 model Cpt with improved performance could work.