It'd too expensive to summon and 10 population slots make the 120mm mortar a tremendous manpower upkeep. So keep the stats but reduce at least the popcap.
In 1v1, OKW.
In 2v2 Wehrmarcht, not sure.
And in random team gamethe faction with best artillery or best Tank destroyer, so either Soviets (katyusha)/USF (Jacksons) /OKW (Walking stuka)
3 rifles is still the way to go before tech tho. I would suggest to reduce the cost of both lt and cpt to 200mp 30fu and the reesearch time from 70 to 40 sec. Then add the extra time and fuel cost to the platoon and company post upgrade (from 20fu and 20 sec to 25 fu and 50-60 sec).
First I'm not a USF fanboy so buzz off, next his solution is dumb, issue right now is lieutenant to M20 to stuart is too fast : so you either increase build time in either or all of those, or nerf tech cost by 10 fuel and/or some mp.