Dear Nancy -
With an ultimatum you give them all the power over your life and your choice. They are the ones who get to choose. And even if they say they'll go, they can just change their minds again. There's no way to force them to do anything. It gives them all the power and makes you helpless.
I would not rely on ultimatums for anything in life. Your health is too important. *You* should choose what is best for you - and you can't force anybody else into playing roles in that. They won't do it. People are just that way. They resist being controlled. I just can't see your parents acting logically or rationally about an ultimatum.
For all we know, they want you out of the house so they can proceed with their bizarre plans. So you are giving them the perfect reason to say "Yeah, go ahead."
You should choose what *you* want. You should do what is best for you, and it can't depend on your parents doing or not doing something. They are uncontrollable.
I'm all in favor of drinking water rather than Pepsi even right now
You can't guard the house forever. No matter when you leave, they could decide to burn it down. If you worry about that, you'll be stuck there forever. You've done all you can on that issue. Now either they'll listen or not. But remaining an eternal watchdog over the house just isn't a good option.