Lucy is a chub, but she's a pretty chub!
Hey, there's more of her to love.
As far as weight goes, this is sad...I do have a few suggestions, but I'm sure there is nothing I'm going to add that you haven't considered.
*A low calorie diet, fed in two meals, one in the morning, and one in the afternoon. I would only offer her 1/4 c in the a.m., and 1/4 cup in the p.m., of lite CANNED food, that contains NO grains...which have been known to create weight issues in cats. In her case, I would even consider transitioning her slowly to a raw diet, but this will take A LOT of research and planning on your part, and should be done under the advisement of a raw-friendly holistic vet. Even a partial raw diet could help with the weight issues.
*Do not allow her any access to the food of your other kitties. Don't allow her to "sneak" food from any free-feeders you have.
*Experiment with as many interactive toys as you can...get her moving, even if the play sessions are no more than 5 minutes at a time...you can slowly try to add on more minutes every few days, and offer 3-4 different play sessions each day.
*I don't normally agree with any cats outdoors, as I feel that most cats should be indoor only for safety reasons, but in Lucy's case, I would do some researching about walking her on a harness 3-5 times a week. She needs the movement, and perhaps could learn to walk with you for a half hour to an hour each day. This will also take some reading and learning on your part if you've never walked a cat before outdoors...I've never done it, so I can't offer you any great advice on it. However, there is lots of reading that can be done on the subject on the web, I'm sure.
Anyway, bless you for caring enough to help this little (or big?) girl! She looks like a sweetheart, and her markings are gorgeous! You can do it!