I just finished reading The Plant ( absolutely loved it ) and I am suspecting Stephen King might have finished it just not released it for a reason or other. It just seems very strange that he would leave those characters in the place they are, when the story is reaching such momentum. According to him, it's almost like the characters are telling him the story, he sees events unfold in his mind and he just writes it. It seems so strange that he would have decided to stop it like that. Just a thought...
Afraid not--it remains unfinished as he's lost the muse with that one. It's just as you described but the characters stopped talking to him, so the story remains where it ended. There are a number of other unfinished stories in the file drawers--it's just that those have never been made public.