I'd look up diatomaceous earth for dogs. I give it to my pup as a natural dewormer but it looks like it's helped for a lot of ailments. Food Grade though.

Yes, sometimes we just have to let go of them. I did with my elder cats last year, and I cried a river, two.

But I like to fight diagnosis, so I believe you can prolong your dog`s life, make it good. First thing is the selenium content of the kibbles; if it is right; 0,8 mg of selenium per 120 gram (serving size)
that is a whopping 800 mcg per serving for a small dog, for a larger one, 3 times as much??

I would drop this food all together, unless they have done a mg / mcg mistake, happens quite often; or that the number should be 0,008mg. This happens often too..
I would give her raw food: cartilage, raw bones, raw fat. Liver and other tripe. As raw as possible.

Then add a "ton" of iodine, for a labrador 50-100 mg and no selenium if the listing of the food he has been given is right. Selenium has a loong half time, years, Iodine just 24 hours.

Then add borax, the bone fixer; also adress the fluoride, and the pain!. Used as a cancer remedy these days, boron.
Magnesium adress the bone pain as well.

Also; the homeopatic remedy Calcium Phos is a bone pain killer, you can give it daily in her water for the rest of her days.

I kept my little dog alive for another 6 weeks, but I know exactly how you feel...and today we put her to sleep. It's something I consider to be the hardest thing in my life I've ever done. I'm 51, done a lot of things in this life, seen a lot of death...this was the toughest. I truly believe that sometimes we keep our pet alive for ourselves more than it is for them. If you can power through your emotional attachment, you probably know that putting your friend down is the right thing to do. Not the easiest, damnit, I just did this 2 hrs ago and I'm incredibly, deeply in a state of sadness...but my brain knows it was the only thing to do and I needed to be strong and do it for that little life. That little life that made us all so happy for 12 1/2 years.