I never had a chance to read the play before today, and it is very enlightening. I can see how Mr. Rushworth might not have initially seen why there would be a problem with Mr. Crawford playing Fredrick to Maria's Agatha... after all they would be mother-son, not "lovers" - and Mr. Rushworth would play the Baron who would ultimately be with Agatha (but not until the end!) Reading it helps give MP's description of rehearsals more context. :)