@topdrop, in your scenario, if it was in CAS then ATC would have everybody on QNH.
If A had been cleared above TL and had set 1013 but subsequently had been "stopped off" then ATC would ensure QNH is passed again.
Any incorrect altimeter setting has potential for a loss of separation, full stop.
Personally, since the introduction of RVSM, I very rarely noticed any Mode C with an error of 100 feet let alone the accepted tolerance of 200 in any commercial aircraft,