Am so so sorry that this is so tough on you. Its really not fair...

Am shocked over threatening to kill herself, and setting fire to the kitchen. There's no way of saying this kindly, but she has big mental health issues. Surely there is no age limit on mental health? Can you talk to your doctor? Her doctor? Do you have a mobile phone? You could record the next outburst and play it back to your doc or someone else who's previously refused help? Keep a diary too, of any outbursts.

If she's just manipulating you, and this makes me think its the case because she acts normal to other people, then you only really have a few choices. Keep trying to get help, insist, try threatening to walk away to social services -my mum who was a carer of her mum in law threatened to commit suicide to social workers - it worked. My mum had no intention of it, but the authority had a responsibility of care, both to my grandma AND to my mum.
Or you carry on as you are, or you do indeed walk away. I know the last you'd never do, but you are entitled to some sort of life of your own. I could say there is no point in two lives being ruined, instead of one. There would be no shame in that, you've already done more than most people would do, ever.

Again, I really really do feel for you, such a hard place to be in. Hugs coming over the pond.. x

ps - I am sorry that things are so tough over in the States without insurance. I find it hard to understand that everyone is refusing you help :-(