Hi Jonathan,
I have fibromyalgia and transition has actually helped me in a lot of ways. My doctor and I talked at length about my condition and how I was doing prior to starting HRT. She almost always starts people on a low dose, and I think that definitely helped me ease into things. I'd encourage you to try to find a doctor who is comfortable handling all of your health concerns, including your transition. I think this was instrumental in helping me be successful.
(1) In general: I was diagnosed with fibro when I was 21. I had at least one flare per month, sometimes more if I was stressed. I started T at 25, promptly lost 20lbs and dropped my body fat by 10%, and didn't have any flares that weren't seasonally affected. I've been on T for over a year now, and the only flares I've had were weather related. I still have some very mild aches and pains when I'm overexerted, but I'm no longer at the mercy of my condition.
(2) Migraines: I had migraines with my flares, so in my experience, these have gone down. Like I alluded to above, if I get any, it is mostly related to the weather/barometric pressure. I can't control the weather, but it's a huge improvement over what it was, so I'll take it.
(3) Binding: I was binding for almost a decade by the time I got top surgery. It was miserable, but I had awful insurance and no money until very recently. I'd be very gentle with binding. I have some numbness to my sides/back from doing it so long term. Top surgery was actually a huge relief for me. I'm not sure how long they would have you go off your meds for surgery. For most things, I had to stop them 2 weeks in advance, but could immediately restart the day after surgery. Personally, I'd rather take a more intense bout of pain at once than have to deal with binding in the long term. It would be a question for a surgeon though. I'm not 100% on what medications need to be stalled.
If you have any other questions or if I can clarify anything for you, feel free to ask.