Be positive (I know thats hard under the circumdtances). Explain your increased responsibility, plus your time in service. Exp lk son how it probably wouldnt get done without your expertise. Do not bad mouth your present boss. They may ask you questions about your boss, be honest give accurate details, but don't make it seem like you want revenge or vengeance against your inept boss (Which you rightfully have to feel). Explain how you are short staffed and have been carrying the ball on the weekends. I do not know if this helps. I hope so.

Just don't wiggle your finger and say "We have a problem Mrs. Torrence". Trust me, it won't go well.

I think you should call and make an appointment to talk to the regional manager. Know what you're going to say and try to anticipate all possible reponses. I would not mention the pay of anybody else. I think you should just first ask for the raise, and then if necessary, present your case for why you're worth more pay. I wouldn't mention the manager at all unless you're asked about her.

Tell them you are taking on more responsibility since you are short staffed and deserve a raise. Don't trash the manager but answer questions honestly. Even if you get a raise start looking.