[Msquared]

This isn't a tire problem: it presumably wasn't doing this before they aligned the car! What specs did they actually set it to, front and rear? It sort of sounds like they cranked a shitload of toe out into the rear. Or maybe the tech forgot to tighten one of the eccentrics and so one toe or camber adjuster is moving around as you drive it.

Camber could be advised differently depending on how you plan to use the car, but you should be aiming for between 0 toe up front and 1/32-1/16" toe in at the rear. I don't know enough about the steering centering reset to say if this could be an issue. Either way, it shouldn't matter that it's a Cadillac dealership: every car they make plus the ATS is/was based on this same platform, so it should look familiar to them. Get the alignment specs and lets go from there.