I just have a couple comments.
In the video, she looks okay starting out but at the 2:00 mark, i can see some off steps behind.
The uneven muscle development is a common symptom of EPM. If she has had falls, especially on good footing, i would suspect that is an issue. Please get her tested for EPM. I watched a video on thehorse.com with EPM in a thoroughbred. It can present with many different symptoms. Uneven muscling, and slight lameness issues are some of them.
Next, her conformation is a mess. Long ankles, very upright shoulder angles. The left front ankle looks dropped in the picture. This could be from long toe, low heel, or this could be something like the beginning of DSLD. That said, my friend has a horse with absolutely terrible conformation and the mare is as sound as can be. Given her conformation, she is definitely not something I would consider breeding, and shame on the people you got her from.
If she doesn’t have EPM, i would suspect some sort of traumatic injury to her pelvis at some point, perhaps as a foal. I’ve seen many young horses but never any with that degree of muscle loss. Were your pictures taken with her standing square? As an uneven stance may make her look even more lopsided then she really is.
All things considered, she doesn’t look that lame. She may be a horse with physical limitations. I would probably just stick to flatwork with her. Eventually this will cause problems down the road. Most of my horses have physical limitations - it’s just a matter of management and not forcing them to do something they can’t.
Her canter looks bad. I suspect her canter will be terribly uncomfortable under saddle. She doesn’t look like she’s moving correctly behind.
As for bits, one of my mares hates broken snaffles, but rides really well in a mullen mouth, or even a pelham or kimberwick. Nothing wrong with a Bosal either if she’s well behaved in that.
Good luck with her. I’m not sure physical therapy will help. I would want her fully checked by a lameness specialist and diagnostics done. Before asking her to work, make sure you know as much as you can about what you are dealing with.