If you're fairly certain you'll be fired (and it sounds like you are) wouldn't this be the most sensible solution?
When asked in interviews why you left saying you quit usually goes over a lot better than saying you were fired. And explaining why you quit would certainly not be considered a poor reason - at least not for the companies you would want to work for.
Companies really shouldn't pry any further when there's good reason to believe you're legally obliged to shut up. Plus refraining from the opportunity to rant about their dubious practises and coverups also implies that you're a sensible professional with high standards rather than a troublemaker.
The possibility that your most-likely-soon-to-be-ex-employer might feel compelled to offer you some kind of settlement in return for you agreeing not to mention alleged non-compliance or unfair dismissal to anybody else actually probably still exists. But you ask your counsel (or two or three different lawyers) about that, not HN.
Plus companies will often terminate you via a technical layoff, or 'position eliminated' instead of 'termination for cause'.
Don't give HR at the new employer any reason to wonder about what happens after you're hired. This vague stuff is not cool. Try, "My girlfriend and I were ready to moving to Portland for her job as a biochemist at xyz but it didn't work out (provide more detail here) so here I am looking for work back home" or something similarly tragic but obvious one time only concrete specific event. Or your position was in line for a forced relo and you really like (insert name of city you're interviewing at). Claim to be downsized, how are they going to know? Perhaps you have a terminally ill distant relative who did in fact recently die so now you are looking for work thats more challenging now that you're not distracted.
Also provide future, unverifiable reasons. I intend to attend the university of XYZ masters program next semester and your office happens to be down the street from the university of XYZ so I was thinking ... And if you end up not attending, well so be it. My girlfriend/mom/dad/bro/uncle is planning on moving right down the street to minimize commute, and we are a close family and oh, look, you're right next door.
The laws and lawsuits are so strict that you can pretty much tell HR anything and your former employer will only provide dates titles and salary, or you'll be collecting a large lawsuit paycheck from them. If you're going to tell a story, tell a good, believable, detailed one.
Note that if you file a lawsuit, most HR types can search the court record and will ask you why you sued your former employer for wrongful termination when you claimed in the interview that your girlfriend was moving to Austin or whatever. So settling out of court, assuming there is a settlement, is in everyone's best interest not just the former employer.
Look on the bright side, better to escape without legal action, than to escape with a "sued his employer" in the court records, which is better than "named in felony proceedings for falsified corporate accounting records" or whatever they were setting up to frame you for. You're in a bad spot but you've avoided so many worse possible spots.