Hey zk,
it sounds like a difficult situation you're in -- I'm sorry! Just a few things: you're not alone, ok? We can relate and know how it feels. Now that's not much, but it always helps me a little when I'm down.
Do you have a therapist? It sounds like you're dealing with a lot of stuff, and it would be good to get some support there. If you have one, can you discuss coming out to your parents with them? The first priority should always be staying safe, but if you think that your parents will not kick you out or get abusive, it's something to consider.
The weight issue... I can totally relate. I've been underweight for as long as I can think. Many different reasons, but one thing that kept me from even attempting to gain was knowing it would be in the "wrong" places. I have finally started working out to gain muscle. I'm pre-t, so no miracles, but I've managed to gain about 1.5kg keeping my body fat roughly the same. I'm ambivalent concerning the changes (legs look more misshapen in my mind, but I love the changes in my upper body). Working out boosts my mood and confidence. Maybe this is something to consider.
Keep on posting here, we will give you all the support we can [emoji4] .