Hi Quinn,
I can only answer from my experience, but I'll have a go. I've lost quite a bit of weight twice in my life. Pity about the bit in the middle.
I believe weight loss is mostly psychological. If you're in the right frame of mind its easy, and in the wrong frame of mind you pack it on instead. That's my experience anyway.
HRT will slow your metabolism down, but it may also provide you with the right frame of mind to lose weight. HRT and transitioning may provide you with a goal and motivation to lose weight. I'd assume there's a reason why you overweight in the first place, and its not because you wanted to be. The answer you your question 2 then depends on you.
You may have problems getting HRT while so overweight, but I'm guessing here.
1. I don't know but I doubt it makes much difference.
2. Above
3. As for 2.
4. As for 2.
5.
The first time I lost about 50 lbs in a year, with dieting and a lot of exercise. No HRT. I was very serious about it and it was a major effort.
The second time I lost more than that in 6 months, all dieting and no exercise. Full transiting HRT. Tons of motivation. And basically starving myself (very healthy starving though).
From that I've concluded HRT and exercise are not a major factors compared to diet, and diet is completely dependent on my willpower and mental state. Exercise certainly made me look good though, and I felt great about that. Obviously you're going to be healthier if you diet, my point is only about weight loss (and you're going to be way healthier doing that).
I think it would be very difficult to lose 100 lbs in a year, especially without starving yourself. The second time my rate of loss was above that, so its possible, but really hard.
For me the technical side of weight loss is easy. I'm good at finding things out, and I know how to do it. The psychological side is the hard bit. Its probably something you should discuss with your therapist when you find one. And discuss the weight with a doctor to work out what you can safely do in terms of diet and exercise.'
For me the answer to all your questions would have been start HRT immediately, but you're not me and that might be the wrong answer.