Thanks so much for all of your help, and your good wishes as well.
The vet did not do any tests. She said we could do blood work and urinalysis and so on, but she and I decided to try the Rimadyl (twice a day) first. It does seem to help her, in that she has brighter eyes and is more interested in playing tug, and seems a bit stronger when she does.
I am not sure if I should have other tests run or not.
She did eat "elevenses" the other day, finishing the other half left from breakfast.
But then, another day she did not. On days I am home I will continue to offer it.
I just keep trying things. I always heat up her food so it is warm. This morning breakfast was rice, boiled steak with juice and the fat, and two scrambled eggs, and she ate all of it right away. 
Unfortunately, that doesn't mean that combination will always work. And I have found that if she doesn't eat one breakfast, most often she won't eat anything else I offer to her at that time. Sometimes she turns down cooked chicken, or liverwurst, even. I offer her various things at random times of the day, now, as well.
I sometimes can give what she doesn't eat to the other 2 dogs for their next meal, but of course food also gets wasted. Not that I care. I just want Kit to eat so she can stick around as long as possible.
It's just a guessing game/ gamble every morning....what to try this time. I keep trying to sneak in a little fat, like cooking the eggs in butter or olive oil, keeping all the fat on the meat I cook, so that maybe she maintains her weight. I should try pancakes. I make mine without any wheat, using buckwheat and rice flour, and I put extra eggs in them.
If she goes as low as 40 lbs it will be time for more intervention, I think. That is when I will do tests, and might try the mirtazapine if the vet thinks it is a good idea. Unless I decide to do it sooner than that.....
Again, thanks for all the support and good ideas. It helps a lot.
D'Elle
and family.
Left to right: Kit, Jester, Boo, Digger

"You gonna throw that?" --Jester: 2001 - June 24 2016. Remembered with much love.
"I'm grouchier than you are" --Kit
"I love everyone!" -- Boo
(Boing! Boing! Boing!)--Digger
And not pictured, Benjamin the cat, who thinks he is a small border collie with superpowers.