What is the best prop location for distributed propulsion - pusher, tractor or blown flap for an ultralight?
You are not wrong to go with tractor config.

In my opinion the best location of the propeller is pusher. The reason is that there is no airframe behind to interact with a heavily turbulent propeller wash. The thrust is developed on propeller blades alone and the airflow on the wing is the result of wing being thrusted forward in stationary air. It has nothing to do with propeller wash. The wing benefits from the most undisturbed air possible but the propeller gets the wash off the wing and airframe. For ultralight it may not be a big deal because of good thrust to weight ratio.

The tractor solution puts the engine ahead of the wing and is more common. The propeller gets the soft air and the wing gets the propeller wash. Maximum thrust, but not the best lift or drag.

Distributed propulsion means you have more contributors to propulsion.

15mph stall is similar to paramotor. You may go even lower if you’ll consider a gyro plane.