I'll add something to this thread also. My 2000 MGF suffered similar problems to rear shock mounts. A relatively early canned spring conversion, customer lived in rural area, dirt roads etc. I assumed the rally crossing everyday to and from her property contributed to the issue. One failure was the entire rear DS inner panel parting company from the bodywork...and I mean the entire inner panel!! In due course I found that the supplied shock absorbers had an internal bump stop fitted up inside the shielding tube. As it turned out there was a mismatch in shocker travel to vehicle bump stop clearance and therefore the shocker bump stop was doing most of the work and of course transferring those shock loads to the bodywork. I removed the shock bump stop so allowing the factory subframe stop to start doing its job. So far so good. Something to check.
MGF approaching 300000km...if its not one thing its something else.