With the turbo mortar lurking around?
I don't even get a sniper against USF since it got in the live game, too risky. You are better off with 2v1-ing lone riflemens with double grens or rushing down pinned rifles with pios.
Against UKF is a better choice in a way, that they start off with lesser squads that needs to be in cover to be effective so you can poke them till they get bored and retreat or die. But against riflemen that wouldn't work as they perform the same in cover as not in cover and with 5 mans they just rush the sniper. And microing the "shoot a guy with a sniper then run home, then go back and repeat" is not worthy against them in the current state.
Mortar is less of a problem unless you are not in full health (you will survive a direct impact).
If you are only depending on miss micro on Rifle vs gren engagements and ONLY on your MG42 play, then yeah, expect to get roflstomp by a Stuart. I'm not saying get a sniper 24/7, but atm, it's one of the strongest units OH has access to.
"So good people, help me to adapt as Ostheer, since clearly it is all me. Tell me what I can do on these perfectly designed maps against double USF mortar, riflemen hordes, a fast Stuart and either double CalliOP or a Pershi"
1-If mortarS are early, they lack infantry pressure. Try to 2v1 early on, on positions which are not near the mortar.
2-If you use a sniper, you might force a LT tech. If he goes for an M20 you can then proceed to the M20 vs 222 phase.
222s are more or less a liability if they just rush Stuart. If the Stuart is delayed, you can then put pressure on the mortars with the 222.
3-Another answer to fast Stuart is panic Puma.
4-Several CalliOPs are more of a 2v2+ issue. When CalliOPs are called, that means almost none medium tank play for quite a while (more so if they got a light vehicles). Rely less on support weapons (not saying not use them, but don't rely on say a pak wall as your only AT) n slowly transition into more mechanize (mediums) mobile force
5-Pershing: use Stugs n TWP (both the pak and stug version). Pershing play is almost telegraphed. They probably won't get major for a long time and rely just on Stuart n infantry.