It's really tough to know what's the right approach sometimes. What worked last winter may not work this winter
You didn't mention metabolic issues, so that means she does not have insulin resistance, equine metabolic syndrome, or cushings?
If she were to have any of those, the glucosamine is a huge no-no. I think either Devil's Claw or Yucca is off the list of "can haves" too.
The horse I mentioned above, has equine metabolic syndrome. That beget hind gut ulcers and lipomas, therefore no glucosamine, and absolutely no aspirin or Bute.
He is on 99% pure chondroitin/msm, bought from www.herbs4horses.com
and Med Vet Phamarceutical's Hylarin/Boswelia that I buy thru United Vet Equine.
They help him a lot.
He doesn't deal with shoulder issues so I have been wracking my brain trying to think of something for your mare.
I'm just about coming empty handed unless you want to think waaaay outside the box and try either liquid linaments or gel poultices. You would have to clean her shoulders every day if you used the gels. Horse Liniments & Poultices, Horse Care | Pet Meds| Allivet Trusted Pet Pharmacy
I use "Sore-No-More" clay poultice on my fella when his hind legs get to bothering him too much and I also use it on my 17 yr old with torn ligaments and sesamoiditis. It really and truly works but it would be a bloody mess to clean off your mare's shoulders every day, especially during your winter months.
Sore-No-More makes a liquid but I have only seen it in 8 ounce bottles; you could go thru one of those in less than a week.
I think everything you're doing is right except, as someone else stated, maybe too much of all good things. It might be better to strip back and just use what gives her the most relief and know she is never going to get better.
Keeping her weight down (as in on the ribby side
, and keeping her feet well-trimmed are also very crucial. I know you keep her trimming current but if she's a bit on the chubby side, try and get some of the weight off.
What she eats can also matter in terms of aggravating inflammation, or not.
I have often thought somebody needs to invent battery-powered heating pads for horses and some way to velcro them to the horse.
I'd be first in line for one - lol My 26 yo Arab has arthritis in his vertebra from an injury before I rescued him 19+ years ago. My 25 yo, with EMS, has hock/ankle arthritis and has developed it in his rump. My 17 yo has the torn ligaments and sesamoidits (bad farrier who cut him waaaay too short) and he is going to have arthritis.
Only my 18 yo is healthy ---- as long as you don't count mental issues
Hopefully I've given you some ideas to try and maybe even help you come up with something else. Just because nobody else has thought of "it" doesn't mean you shouldn't try it, as long as nobody gets killed