I was looking at this target myself last night! The problem is, as you noted, the mighty gamma Cass... The diffraction spike would come in at an (unfortunate in this case) 45 degree angle if I'm not mistaken? Also, I get a different FOV for P9, based on 2740mm focal length and 23.5x15.7mm sensor?
You are correct. I selected the wrong camera.
There is not much we can do about Gamma Cass. There was a similar problem when I imaged the Witch Head Nebula (IC 2118). There were some streaks from Rigel, but I left them in place as it showed where the illumination of the nebula was coming from.
I think it will work OK. Would you like to submit the job, or shall I?