Update: Arthur has chronuc pancreatitis (which I'd already figured out), which flared up and also caused a bleeding ulcer, which caused his anemia. The pancreatitis caused the insulin resistance and DKA. He is responding well to treatment and should be home in a day or two. He doesn't have cancer, which was our biggest fear. I will have the vet train me to give rapid acting insulin, since that's the only thing I haven't done to head off what are and will always be occasional serious flare ups.
I am so grateful to DCIN and you guys for connecting us and providing support. It's hard when your pet is so sick, but there's no money in the bank