I have an 29 year old appendix gelding. I bought him when he was 14 at that time he wasn’t stiff but bout when he was 16 he did start showing symptoms of being arthritic. He does have navicular (which is common in TB’s, QH’s & Warmbloods but since your mare is acting up with the back feet doubt that’s the cause) and I was dealing with lameness issues for the first 9 months I had him until we found out why. Once it was taken care of he was worked 5 days a week for 30-60 minutes and rarely showed any lameness just a bit stiff in the back until he worked out of it after the first few minutes of warm up. As a side note I my horse made the transition from western to English as well. Is it possible you may be asking her to make the transition to quickly? since the styles are different her body may just be reacting to the being asked to move in a different way.
I am not a big fan of hock injections. I have done both feed through joint supplements (Glucosamine with MSM) and IM injections with my horse. I much prefer feed through since I dont need to worry bout nicking a vein but the IM injections are supposed to be a stronger dose.
If I were you I would try the feed through or IM and use the hock injections as a last resort.