Thank you Sharon,
Based on your description of her gait, especially that she is stiff when rising but warms with activity, I would suspect arthritis in this case as well. Also I would note that her weight does her no favors with this, since it is likely worsening her signs due to the extra weight on her joints. And this may be why we are seeing stiffness at her this point in her life.
Now I am glad to hear that the Metacam is helping. We do find it a great benefit for arthritic cats. And as cats often refuse to be active when they have underlying joint discomfort, it is possible that if we can manage this for her that she will become more active for us and help keep her own weight in check. And this is quite important, since reduced calorie intake is only part of the equation in weight loss for cats (or even for us). So, if we can get her feeling more comfortable, she may be more amenable to play or being pushed by you to be a bit more active.
So, I would advise keeping on with the Metacam at least for the short term. Over time you can try weaning her down or using it only as needed. Further to this, I do want to note that there are also OTC supplements that could also help her joints. This most common ones that we use in cats are omega 3 + 6 fish oils and glucosamine/ chondroitin.
With the fish oil (Omega 3 +6), there are a natural anti-inflammatory and could benefit her joints. When offering this we tend to give 20mg per pound of their body weight and many cats have no qualms about a bit of fish flavor added to their food. And its available over the counter at your vet, pet stores, and local health food stores.
Otherwise, with glucosamine/chondroitin supplementation, this is a nutrient supplement that is available at your vets, pet shops, and health food stores (as capsules, liquids, and even treats). It works by aiding joint suppleness by helping cartilage replenish itself and blocking enzyme destruction of cartilage in the joint. Often we can find this helpful in animals with arthritis. Normally we give kitties 50mg glucosamine + 15mg chondroitin a day per 10 pounds of body weight. So, this too would be something to consider for her.
Finally, since you have had many diet food trials that she out and out refused, you may want to consider finding a lower calorie/fat diet she likes and feed her based on her resting energy requirement (RER). You can do this with her current diet if you wish, but we just need to keep an eye on how much food this does or doesn't translate to. Because if what she eats now is very rich/fatty, feeding her based on her calorie and RER needs could mean tiny meals (which could lead to her stealing from her friends).
Now when we want cats to loose weight, we need to work their weights down slowly. As I am sure you can appreciate, crash dieting is not good for cats (or even for super models). So, we want to wean her down in weight to where we ultimately want her. Therefore, to start, I would advise just feeding her for a RER for 6.7kgs. If we do that and she has been on more calories then she needs, this weight will reduce. Once she gets to a stable weight, we can choose a new target just below this and work her down to that and so on. And of course, no treats, tid bits, or anything else when feeding this way.
Now the RER for 6.7kgs is = (30 x Body Weight in kg) +70 = 271 kCal per day. So, that is her daily ration level for this point. This is all the calories she needs per day to maintain her current weight with no activity at all (which as she will be doing more then just laying around unconscious, anything she does would work to burn a bit of calorie and help her lose weight). Just to note, cats have no fat or carb requirements, but when we are feeding any diet they should be getting at least 2 grams or protein per pound of body weight per day (so ~29.5g per day for her). So, with this information, you can now take that value (271 kCal/day) and look at how many calories (kCal) there are in her food and work out how much of her diet she can have per day and only reach this level of calorie. And this can be used for any diet in order to reduce weight.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,