My flare lasted pretty much the whole of winter. Like you my muscle tone deteriorated quite a bit. I started with light weights and rowing on my rowing machine. I have a mini gym in my basement. I pretty much have to force myself because I do battle constant fatigue. To climb stairs my legs literally burn. Because I have a past history of running I wanted to get back to that knowing I wld be much slower. I started with the mindset of alternating walking and running. I have a walking path around my home so I utilized that. I've graduated to trails now. Again, I hear you that this is something you can't do. On the flip side, I didn't think I could either as I just spent months in bed. This teaches me that we may be able to do more than we think we can.....to whatever degree that may be.
The ball I'm talking about can be purchased in any fitness store such as Sport Chek. Google it on line. It's blown/inflated. The user can lie it, supporting the lower back and perform crunches, or light weight flies etc. The ball comes with an insert with pictures and different skills to perform. Nothing too challenging or difficult. The ball is designed to isolate muscle groups without placing little stress on the body.
Regarding the bands... Always sit or stand with good posture. The band can be placed either around the ankles or knees. Direct you leg laterally for the reps you want, then behind, then forward. Only direct your leg as far as you comfortably can without arching your back. Proper posture is key. The movement can be very minimal. You should feel your thigh muscle working. These bands can be used for upper body as well for stretching. Again, Google or look at YouTube for ideas. Remember, always use proper posture. Learn to differentiate between the Fibro pain, the "good" pain that comes with working out and the "bad" pain, when we over exert ourself. Yes, toning your muscles will make them stronger, and you will also be strengthening your bones.
I admit I can over-do it. I am slowly learning my limits and my Dr always reminds me not to over-do it. She knows I have an active workout history. Like you I am in my 50's, 59 to be exact, turning 60 end of August. Because I look after myself, thankfully I don't look it. I guess I'd rather hurt by working out than hurt while living a sedentary life. The key is to know when to push it and when not to. I've hit the wall many times setting myself back but, by doing this I am learning where my limits are. Another point you mentioned that I too worried about is the "irreparable damage". Fibro isn't terminal and our muscles have no literal damage to them. We feel pain and fatigue, but our bodies still can move and work. Albeit not the same as pre Fibro, but, we can still move. The key is to find out how much we can that is unique to us. A few years ago I complained to my Dr how my stamina wasn't improving regardless of my workouts. I could do a boot camp, by far surpassing others 20 years younger than me but struggle to get out of my car after driving home from class. What's with that????!!! My Dr reminded me that my stamina will most likely never return, so I had to adjust my thinking and approach. So, I do what I can with the knowledge and goal of trimming up, (I'm 118 lbs), feeling I'm accomplishing something, enjoying the positive endorphins exersize gives, my man is encouraged by my efforts, (I'm sure he must get discouraged seeing me sit or lie down everyday), and I'm reaching goals I never thought I could. I have a 10k I'm training for in August with my sister. My mindset is I will run, walk, and complete it. My husband will be waiting at the finish line. I want to see his smile as I cross holding my sisters hand.
In conclusion I encourage you to test your limits. Start out slow but do SOMETHING. Push any discouragement out of your mind. If you walk for a total of 5 minutes then stretch, conclude that you walked 5 minutes more than you did before!!! Celebrate your successes. I will be here celebrating with you.