Never flew MDA, no dog in that fight.
However it seems pretty simple. USAir tried to convince everyone that MDA pilots were not Mainline. However the simple fact that there has never been a MDA certificate seems to make it a closed case.
Mid Atlantic never existed in the eyes of the FAA. Mid atlantic aircraft flew on USAirways certificate, just like Metrojet did prior to that. Play musical chairs all you want with the screwball working agreements and payscales but the simple fact remains that MDA 170's flew on USairways certificate, were controlled by USAirways dispatchers, and were maintained by USAirways maint. department.
Saying MDA was not USAirways is the same as saying the west operation is not USAirways. Different sen list, different payscale, no mixing of crews, but one certificate.