I watched the video. She was sounder than I expected for everything you shared about her. And I love her name. 
I don’t see DSLD/ESPA, c-spine, or anything that bad just yet. I see a young, unbalanced, somewhat anxious but also somewhat lazy horse that doesn’t trust contact or pressure, and isn’t confident pushing from behind. I see a somewhat shorter RH stride with her wanting to bring that RH more past the midline than with the LH. You can see when she picks up her head over the pole, it’s when she has to push that RH off. Does it hurt? Or is it pain memory? Only you know her well enough to know.
I would not fuss about finding a saddle right now. She is 5. I can’t tell you how much my WB changed in size between her 5 y/o year and now. I put off buying a saddle for her until I knew she was done growing - so, last spring. I hear you on being frustrated that the saddle you had checked over 3x is now being told not a remotely good fit. BTDT. Take your current fitter’s words with a grain of salt - they are so quick to put down a past saddle fitter’s work. The only person whose opinion truly matters about the fit is Forza. And keep in mind six months ago, her atrophy was much worse wasn’t it?
I know you want to ride her, but watching the video and all you shared, I think this horse would really benefit from a few months of long-lining, trotting and cantering over raised poles - mixed in with lots of hacks (bareback, if you have to!). You’ve done a great job addressing the pelvic asymmetry. (Side note: I have a horse that broke his pelvis in three places, I know the journey. It’s hard. Happy to share what’s worked and what hasn’t.)
I’d be putting a bellboot on that RH each time you work her. I did this with my pelvic-break horse and it helped him remember to use that hind.
Given you know she had some sort of catastrophic thing happen to her hind end, I’d be working more on things that build hind end strength. Raised cavaletti are great for this. And, assuming it was some kind of break (to have caused that degree of atrophy), expect this is a multi-year project. Don’t get discouraged that it doesn’t all improve right away.
Your situation is exactly why I take pictures of my horses front, side, and from back - at least twice a year. It takes some time to set up right for the photos, but you would be surprised how easy it can be to miss something that seems glaringly obvious, because you are used to the horse. Start taking photos if you aren’t already. Include photos that show the feet so you have an objective “goal post” each year to refer to.
Your riding is great. You are very accommodating with her. But don’t let her train you to ask for less.