Wow! That's a tough call to make, however, based upon what you said, I'd say you don't owe them a damn thing. You've been a team player, the quentessential employee. They couldn't asked for more. They did...but in the wrong manner. I'd beat them to the punch...especially since you have another job offer. As suggested, if you believe that your employer is going to fire you, then I would go in ahead of the scheduled meeting and resign. Take the high road.
Btw, not a sob story. All the best.
atp