Hello and Welcome!
So sorry about Smokes. I have no experience with leukemia. Wanted to send along prayers.
We home senior labs so Molly's situation is not unfamiliar. I know you realize getting her weight down is most important. I do not feed senior foods; they have so many fillers. Our Mardi is 11 and does very well on Fromm whitefish. It has less calories and I feed less, only about 1 1/4 cup per day in two feedings. I add frozen green beans (you can used canned, no sodium) and mix in a small amount of frozen fruit (no grapes). Mardi has one very bad hip and a bad knee on the opposite leg so she has very limited exercise tolerance. Even though, she gets her daily mosey, sometimes only to the end of the driveway and back. She loves it. I'm very careful with treats, really limiting them. No table food, unless it's plain cooked veggies. Also add 500 mg of vitamin C (the human chewable kind) and Cosequin D daily. They are chewable tablets so they get tossed into the morning meal. This last winter was very hard, cold, snow, ice, so Mardi did not get out near as much as usual. Even so, with strict control of her diet, she actually lost weight on the Fromm. It's a good quality food, grain free. With her eating so little, it goes a long way. If you change over, do so slowly, over a week or so, so she won't have an upset tummy.
I also do range of motion exercise. With Mardi laying on her side, I get down beside her and start with patting and talking, getting her relaxed. Then, with a hand gently on her shoulder, I move her leg forward and back, very slowly, very gently. It takes a while for them to get used to it but once into a routine, they love it. It's our special time. Only go as far as comfortable for Molly, the range will increase over time. I work each joint on the leg, gently, back and forth. The same routine for the back legs.
Our Melody had lower spine arthritis but I kept up the same routine. She was on Tramadol to help with discomfort. Also had a walking harness with a handle to help her on the bad days.
Please don't hesitate to PM me if you have more questions. Think you need about 5-10 posts to use that function. It's easy to do; go to the picture section and welcome section. You'll have enough posts very quickly.