If this is * fait accompli* and it is going to happen, then by all means, I would involve HR, under the advice of your counsel.
I would tell HR that you're being punitively punished for bringing up an ethics violation with your boss.
I would tell them that your counsel has advised you that you have certain rights, and you expect to be treated fairly.
I surmise that will force them to offer you more to get you to leave quietly, or maybe force them to investigate the asinine behavior of your boss, assuming we perceive the situation correctly.
Maintain your professionalism
Seek independent legal advice
Avoid HR like the plague
Assume that you will be fired. You cannot win.
Start looking for a new job immediately.
Document everything for any legal proceedings later.
Ask colleagues you trust if they would provide you with a reference once you have secured new employment.
Do not fear leaving your job for another. Opportunity knocks.
Good luck and remain positive. Don't let this get you down.