MGB281

I would take it back and get them to get the rear camber correctly adjusted. Yours being a 2003 car has adjustable lower links, that is unless you have fitted the later fixed length ones. The adjustable links could be rusted solid which may explain why they haven’t done it though. It’s a bit of a pfaff to get it right because adjusting the camber alters the toe in so they have to adjust both simultaneously. If you set the toe then the camber the toe will be incorrect again, it’s a case of keeping making small adjustments until both are correct. Having said that I don’t think the rear camber would have such an affect on the stability of the car, I am sure that you have play some where.
A few years ago my middle son who bought a Ford Ka to flip, the seller told him that he had hit a kerb and bent the front suspension. All new parts were fitted but the car was just like you describe, the seller gave up and my son bought it with the defect. The first thing he did was to get the tracking done but there was no improvement, he had the tracking done a second time and still no improvement. Neither establishment could offer an explanation. One day he brought it over to me and I jacked the front end up and using a pry bar found one of the bushes was perished. I presume the shock of hitting the kerb was transferred from the offside to near side which caused it to fail. A new bush cured the problem, I suggest doing the same with a pry bar to see if you have any play.