First, good luck, and don't sell yourself short.
If Miss Chiponhershoulder asks, white lie it, so you don't get negative feed back. You may have to give your boss a heads up about this as well, just in case.
I would simply tell her you asked boss if you could "cold run" an interview. Tell her you know it's too high a position for you, but this way you can get feed back without having to be under pressure of an actual interview.
*Sorry I missed your update.
People with attitude problems don't think they have one, and no amount of telling them they have to change it does any good, anyway. They think it's every one else's problem.