First I wanted to congratulate you for forming what must be the longest thread ever LOL.

A short leg may not cause a person to lean. it can cause the pelvis to tilt lower on the short leg to make it touch the ground. what that also means is that the junction between the pelvis and lumbar spine is not flat, which can cause the lumbar spine to tilt to one side also.  neither condition is desirable from a performance standpoint.  

i don’t have anatomically different leg lengths, but i had one leg shorter than the other when i first went to a PT after i began training for triathlons. he x-rayed me and showed me the pelvis tilt to one side, and the lumbar spine’s curve to accomodate! the fix back then was orthotics, but it wasn’t a fix as much as a crutch.  now i  know there are root causes to be addressed and i can manage it now, even though i still have uneven leg lengths occasionally.

i’m curious – what were you scores in the FMS? also did he give you any corrective exercises? i’m not sure clam shells are optimal. Xband walks are better, but there may be more effective things you can do to help activate your weak side.