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Ok, from a 2v2 perspective
Walking stuka:
If you want to make it more usefull in 1s and not so powerfull in teamgames move it to t4 and add mortar ht in a t2. Therefore there will be a earlier counter to units in buildings and support weapon spam (effective but not insta wiping entire army). Stuka would be a more powerfull but harder to get arty for the later game.
Calliope:
Can't you just make it like Sturmtiger? Cannot move while reloading (and reload must be order manually) other option is to for example decrease the armor while firing and put it behind tech. Problem with this unit is that hard to counter, cannot be counter by barrage, dive is risky becouse of high HP of a unit, ju87 loiter won't kill it in 1 volley (you may say it cheese and abusing but this combo flare and loiter is enough ammo investment - and also need a good cooperation). Changing it to katiusha type of fire won't hurt also.
Not saying all changes must be done in same time but there are best options imo.
Firefly and JP4s are okish imo, remember those units can be easily flanked and you already made changes in cammo system. Tulips costs 100 ammo so it's fair that they can immobilize the tank for a sec.
I would argue if JT/Elefant changes are really needed. Topic is deliberated
Aura effects
We should remove/reduce the effect of the following auras on teammates:
- Soviet forward HQ (just make this ability like 3CPs and it will be ok)
- P4 command tank
Remember this is a teamgame, so a team gameplay, a cooperation should be rewarded, you cannot remove any buffs to your teammates becouse it kills the teamplay spirit.
Other ideas sound decent, should help but tbh if you want to make teamgames more balance just change the map pool becouse it's horrible (at least most of it).