I think it's safe to ignore both of these factors at this scale. 1) The effects of gravity is bounded and negligible on the surface of the Earth. 2) (I suspect) the wideness of the launch angle and hoop size are probably directly proportional, so you could shape the back board based on a direct impact to get a point to center the hoop on. The hoop can only get so big and still be considered reasonable.
The effect of gravity on the surface of the Earth is hardly negligible!
And your intuition on the launch angle is just flat wrong. In fact, I believe it's one of the primary predictive factors in a shots success with a normal backboard!
