Weight does not matter as far as FMS goes. I have lost the same 50 pounds of weight twice since I have had it, it is gone again, and it did not make a bit of difference in my pain. I only have FMS though. It does make a huge difference in how I feel globally, not with the pain though.
However, it does matter for your knees. Every extra pound of weight puts 4 pounds of pressure on each knee. Every time you stand up it does matter.
You CAN lose weight. Find a form of exercise that you enjoy doing. For me it was belly dancing, lol. I am 62 and dance in the TV room where NO ONE can see me. I dance along with DVDs on the TV set, every day. I do an hour a day, every day. I have done it for 2-3 years now. I don't diet, but I probably should. I lose a very small amount of weight each time I see my doctor. A couple of pounds every three months.
You can exercise in a chair if you want to. There are DVDs and stuff on You Tube for everything nowadays. You don't need to lose a lot of weight to make it worth your while.
You can do what you like, but change the attitude you have about losing weight. Do a simple calories in and calories out thing. Write down everything that you put in your mouth for awhile. See a dietician for portion control and exercise advise if you need it, I learned a lot, and I am already an RN. I only saw her twice.
I would discuss your fears about seeing a PM doctor with your pcp, then screen them on the phone, there are a bunch of strict interventionists out there that you do not want to see. There may be something out there that you have not tried that could help you, besides losing weight. It is not an either/or thing, try both....
Keep in touch.
Take care, Annette