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Sorry to hear Fluffy has such problems! What pain medication is she taking? What food are you feeding her? Any change in her appetite or drinking? Is she using the litterbox ok?
Sorry, I see she is on Metacam for her pain. That is good.
When contemplating humane euthanasia, quality of life is the main thing to consider. I look at things like appetite, hygiene issues (grooming, using the litter box, etc), interaction with owners, and general personality (does she act like "Fluffy"?)
Here is a good website with more info on determining quality of life:http://www.VeterinaryPartner.com/Content.plx?P=A&A=1523.
It sounds like Fluffy doesn't always have the best days. I think it can be helpful to have 2 cups--one labeled "good days" and one labeled "bad days." Put a penny in the appropriate cup each day and when the bad days outnumber the good, then it is probably time to let her go.
A "good day" would be one where she is up moving around (and 15 year old cats do sleep a lot, so not necessarily a good indicator), eating and drinking normally, using the litter box, and interacting with you/happy to see you. A "bad day" is one where she missed the litter box, seems to not care if you are around, doesn't want to move around.
A few things to consider: the vocalizing can be due to hyperthyroidism. This disease is fairly common in older cats and is an overproduction of thyroid hormone. Generally these cats are hungry and losing weight, but if she is really obese, you may not notice weight loss for awhile. They also tend to vocalize more. So having her blood tested for an increased T4 level, as well as checking out her other organs, would be a good idea.
It might be worthwhile to have her shaved down and get rid of the matted fur. This will make her feel better overall, without mats pulling on her skin and make it easier for her (and you) to keep herself clean.
Not sure if you would be interested, but acupuncture might be something else to consider. It can be very helpful for pets with arthritis.