I've been trying to cope with a very abusive and mentally unstable advisor. In a recent clash she has been spewing lies and threats. It has become clearer and clearer that despite the four years of manipulations, borderline extortion, and five publications that I've survived through, I will never receive a degree from this advisor. I have decided to accept a job offer and I want to just close things up at the school in the best possible way. If possible, I'll leave a door open by taking a leave of absence for a year to come back to a different advisor.
How should I go about my next steps? Do I email the Department Chair directly asking for a meeting? I have been warned such an email would probably reach my advisor first. Do I just walk into the student affairs department with a leave of absence form? If I should email someone, who should it be, and what should I write there?
I'm funded by an NSF grant, so should I write them now in conjunction or after I finalize my leave at school?
If you're in good standing with faculty overall, a leave of absence justified with a desire to temporarily join an exciting industry option might quite possibly be granted. Justifying it with "personality clash" strikes me as poisonous, so don't do that - at least not in a public request. You say you plan on coming back to another adviser; but it doesn't sound as if you've worked on finding one.
I don't think you've thought this through: you leave for a year, with only a vague idea of coming back to "someone else" who you must believe you'll find while not even on location. If you don't have one, or several mentors behind you (other than your adviser) who you know have always been very fond and supportive of you, you'll have to explain to some other faculty, while away, why you don't want to come back to your old adviser. A person neutral in the matter will naturally want to hear the other side of the story, and talk to your old adviser who is likely to have similarly ambiguous feelings about you. This is anyway the right approach for faculty as the new adviser will not want to act behind the back of a colleague. This has only some remote hope of working out if the new adviser does like you a lot, and it's certainly made harder by - apparently - planning on tackling this while working in industry.
This is to say that - in my opinion, and absent a powerful other ally among faculty very fond of you - you should first switch to another adviser; then consider if you still want a leave of absence, and see if this is cool with your new mentor. If either the former or the latter don't work out, you should accept the very real possibility that your temporary leave will mean the end of your Ph.D.