First, I'm glad the B12 shot helped! It certainly doesn't cause any harm, and if she has intestinal issues of some sort, she may not have been absorbing it from her food, so it's a good idea to give it to her. BTW, if you're ever interested, there's an oral form that I was adding to Miss Kitty's food (there was no way she would have allowed an injection), so if for some reason the injections are no longer an option, but you want her to keep having the B12, let me know and I'll post the link to the company.
My
on the GLM - again, I don't think it's going to hurt to give it to Queen Eva. Whether it helps her arthritis or any other inflammation in her body, that can only be a good thing. And you keep such a close eye on her, you'll know if she doesn't want it or can't tolerate it.
As for the daily prednisilone, she may be one of those kitties that needs a low dose of pred. Years ago I had posted about Dr. Plechner's protocol, where, through testing certain things, he found that many sick pets had high estrogen levels and low adrenal hormones, and by administering a low dose (based on the bloodwork) of pred, it would lower the estrogen/raise the adrenal hormones, which would lower the inflammation and eliminate the disease. When the pets were taken off of the pred, the disease(s) came back. Doctors/Vets freak out about pred because they think of it in terms of high doses, but that's not what Dr. Plechner was doing. He was giving just enough pred to put the body back in balance, just like people who take thyroid medication take the dose that brings them back in balance. All of this is to say that maybe Queen Eva is one of those pets that needs a small amount of pred to keep her in balance.
At the end of the day, as you say, it's about quality of life.