Luckily, you don't have to rely of grenspam to deal with lmg rifles. The lack of grenades with these units and the very low on the move dps make the LMG rifles very vulnerable to the HMG+sniper combo.
Also why do you completely neglect G43 upgrades but put PTRS up as an upgrade for conscripts?
As for your solution: 5 gren double LMGs would be insane. They would vastly outperform double lmg rifles despite being cheaper to buy, cheaper to upgrade, and require no doctrine to get. USF also has no snipers or any elite infantry to counter them. They would also walk over Soviets, who not only have no stock elite infantry, they also do not get stock weapon upgrades. Their sniper is also not that good compared to the Ostheer one.
But seriously though: Wehr has the counters to 2x LMG rifles. Giving them the firepower to just straight up kill the most expensive infantry squads in the game with their own basic infantry will just wreck balance and cause even more power-creep.
What? I think you're missing some stuff; Rifles have nades, BARS are amazing on-the-move (that's the point of them), and the whole "smoke and flank" thing is basically a meme on how to counter HMGs.
Honestly, I don't see 5-man 2-LMG squads being that "insane". The increase in size would increase the base cost (by 56, to ~296, call it 300mp), would increase the reinforce cost by 30 when going from 1 model (90 to 120), and would cost the exact same to upgrade; the BAR is 60m, and the M1918 is 70m, the Gren LMG is 60m.
I'm not asking for the firepower for grens > rifles all day every day. I'm asking for grens to be equal in certain situations, which they should be, due to reinforce cost. Right now the only hope of grens winning vs. 2xbar is by using green cover and hoping that the rifles stay in yellow or worse and stay at range. Vs. 2x M1918 it's even worse, since their range advantage quickly disappears. They still couldn't fire on the move, they'd still be horrible at close range, and their reinforce would still be the highest in the game, and after upgrading, would be the highest costing T0/1 mainline inf in the game.
You'd be right if Ost didn't had access to best HMG in game and sniper.
But they do, no one in his right mind spams grens vs rifles, because thats about the worst thing you can do. Rifles hardcounter grens, not the other way around.
Ost have way more tools at any time then other any allied faction, USF have rifles exclusively at early game. Obviously rifles will beat grens at all stages, that is their intended role and balance.
You can't have wide selection of supporting units AND mainline infantry as strong as opponent, when opponent have ONLY that infantry to pay with.
Then what justifies Gren's 30mp reinforce cost and higher per-model base cost?