Landseer healing ability doesn't affect MG42 Squads from defensive doctrine.
What doctrine is that? Ostheer has these trees: Army Fortress Troops, Army Elite Troops and Army Support Troops and AFAIK there is not a MG42 call-in/unit unlock in any of them.
My bad, I meant MG34 squads from t2.
It seems that Ostheer AT guns dont do any damage to buildings (those on map, that can be garrisoned). I put a group of Strelky into one on angoville and saw it being shot for 5 minutes without destroying it.