I see the point you are getting at. However, I feel that the airflow behind the prop would be "recharged" allowing the canard to operate more normally at a high AoA versus if it were entering clear air. That same charged air would also flow over the rudder as well. If the motor weigh was a hypothetical "zero" then I would put it in the nose. The only reason I wouldnt is because of the possible difficulties in getting it to balance out correctly. I think you are right about the turbulent airflow gathering and passing over the rudder in a pusher config, but I still feel like the airflow from a tractor config would be more energetic even after blowing past the canard and fuselage. It's really just an academic argument because the pusher config is the best option for your application all things considered. I've got a book by a guy named Lennon that will have the answer you're looking for. I'm going to dig though it tonight and report back to you.