If I had a place to go, like you do-I would have been outta there-like yesterday! Yeah, don't be nasty, pay the rest of the months board, but tell and leave. If it were me, I would be out of there the same day I told her. I mean I would arrive with a trailer, tell her that a great place has come available closer to home, I was really sorry, but this is a great opportunity for both of us....blah, blah...load and go. Sure, she may find out where you are, oh well. I wouldn't tell her. Not to be rude, just very vague.
At this point, you have paid your dues and owe these people nothing. Your concern need to be the welfare of your horse.
Most likely, I would also keep an eye on the situation from afar, with the thought of calling animal welfare. Noone has to know, keep it to yourself, but it sounds like her animals need some help.
My only concern is that I believe you said she gave you the horse and said you could keep it there for free. Do you have anything that is written that says you own the horse? That would be the ONLY reason I woudl hang around and make nice with her-just until I got it.