I hope the best of luck to you. I don’t really have anything super important to add, but if it is upward fixation of patella, I don’t see that you said in her exercise routine that you have any hill work, backing work, or caveletti work. Yes, in a reiner’s world, they do travel very collected while being ridden and collection is very important with stifle problems, but you do also need to do specific exercises that are going to strengthen the stifle. There are certain stretching exercises you can do for the horse as well.
Blistering and injections are not 100% “fix all” for upward fixation of the patella. Yes, they do sometimes help but they are not a for sure thing.
My horse has a stifle problem on the right side. My vet did some of her school work at a very prominent clinic in Texas and she said they saw stifle problems all the time in young reiners. Fortunately, with some TIME, many of them “grew out of it” as they got a little older.
It’s still a good step to take your horse to a good facility, but ultimately your horse might just need some time. And a more targeted exercise program (hills work, etc).