If you qualified for that position you qualify for a similar one someplace else that will respect you and your time more. And they can fill your 'shit' position with 2 weeks notice and without you to train your replacement. Good luck!

That... is a good sign to leave.

The company itself seems like a pile of shit OP.

Complain to HR. You got the promotion and deserve it. It's not your fault your old position is crap and no one wants it. That's your boss' problem, not yours, and you should be able to keep the promotion. Or talk to your boss and tell your boss if he doesn't let you keep the promotion, then this is your official two weeks notice. Then he has two positions to fill, not one.

hope a pay rate increase stays

That's when you look for a new job and drop them the second you get it. If they're not going to let you progress now, they're never going to want to.

your boss deserves a fist to the face

I'd look for another job.

Can we say lawsuit?

Yes, this sucks and OP should definitely fight for the promotion they earned and were given or leave but this isn't really grounds for a lawsuit.

tell them to suck it(not literally) and let them fill both positions