Dear Troutbrown

Some of my colleagues may not agree with me on this but I think this problem just might be straight forward. Although i dont see what the real problem is, you havent reported any pain or any functional difficulties...what you are sort off reporting are cosmetic issues.

The area you are reporting corresponds to the belly of the adductor group of muscles. This muscle is innervated by the obturator nerve with origins from the lumber spine, some portions come from the sciatic nerve. you have had various EMG testing which seems to appear anything but normal. You have also had various surgeries to your spine...but you continue to have ongoing atrophy of the muscle group, do you have any numbness? can you adduct your leg easily...compared to the other one? try lying on your side of the problem leg, keep it straight and try to lift it upwards towards the other leg, repeat with othe side...

If you are having problems with this and you have numbness then maybe you have suffered a resected nerve...if its just weak and there is numbness then maybe you are a nerve compression of the obturator nerve...in both cases continual atrophy will occur...and no amount of exercises is likely to help...

so perhaps wait for the Wikipedia reference-linkMRI...in the meantime you can keep trying to exercise that muscle group to slow down the process of atrophy but like i said it is unlikely to stop it...
since you have had fusion of the discs im not sure attempting to mobilize your lumbar spine will make a difference...but you can try this with another physio, the assessment will include comparing the symmetry of both sides of your lumbar spine and pelvis...the initial physio may be right to think the issue is with the iliopsoas muscle..if its really tight, on one side...then it can compress structures from the lumbar spine there...

with the right assessment, you may be able to achieve some improvement with mobilization, stretches and electrical stimulation..but i see a very long rehabilitation period...