I doubt an t0 usf mortar is going to work unless that 60mm mortar is next to useless.
any decent mortar will just give the USF the ability to safety skip grenade and rush stuart. The USF rifleman and stuart are both too powerful to allow for a decent early game mortar.
USF was generally doing that prior to the the mortar being used and they still can do it. 4 rifles into Captain or now 3 Rifles and a mortar into Captain. Grenades themselves are more of a mid-game upgrade when things are settled and you're now in the process of beefing up the Riflemen.
I don't get how adjusting the mortar will make it useless if we were to take the CE mod mortar for example. It can still block off garrisons with smoke and provide some damage against infantry, but doesn't do enough to force weapon teams to retreat by itself, is prone to getting counter-bombed, but does has the ability to keep up with aggressive riflemen. It also won't overlap with the Pack Howie.
Furthermore, giving it less range, means its easier to attack the mortar itself and harder to hit units that are operating in the back aka MGs. You probably won't kill it, but it's much easier to force off and means the USF can't simply keep it back without micro.
I'll say it again, USF needed a mortar, but they did not need an 81mm.
Also, if you're complaining about the Stuart as well, make a topic for that and how USF can generally rush out their light tank the fastest and how it can lockdown the majority of the lights/outright kill them with the push of a button.
Imho, Riflemen early-mid game, even with a mortar aren't an issue. The issue is how fast the USF can pump out their light tanks to bring that shock value forward when faction such as Ostheer might be just getting their first Pak out or their own LV without much room to grab AI units like PGs due to the need to counter light armour and no real method to force the USF to react outside of pissing them off with snipers. OKW, at least during a period of time, can start to heavily pressure USF and they'll have things like StGs coming online along with grenades which Rifles can't deal with unless upgraded or doing it from the start with the Sturm + Kubel push.