Two thoughts:
1) Does being fired negate the non-compete clause? Damn if I know, but I'll bet Don, Rodger, Bert and Price don't either because they are all too used to winging things to check. Confirmed by the way it came up. Don saying to Price "you have the absolute authority to fire us if you choose". Really in the spirit of "Gang, why don't we put on a show?".
2) Betty: yeah she is unpleasant, but also trapped. She drank the too much of the Koolaid for way too long because Koolaid was all that was on the menu. At this point even if she was offered champagne she'd choose Koolaid, because that is all she knows to recognize. And yeah, agency and all that, but it would have taken one hell of a strong character to turn out better than Betty did in the circumstances she found herself in.