What he said.

Even "at will" states have various issues. Denial of unemployment insurance is one -- you may not be able to collect if you resign, or if you're fired "with cause". There's also the issue of opening up a lawsuit against your employer. It's going to be more expensive for them to fight than to settle, in most cases. The "probationary period" may strengthen their case.

Again: I'd talk to an employment issues lawyer. Sounds like you're going to need it. A mental anguish case might be well founded, given what you've presented here.