Curved film plane in pinhole camera actually improves sharpness corner to corner, but introduces curvature to most horizotal (and to lesser extent diagonal) lines, except one horizontal that is dead center on the hole.
Pinhole size does depend on film to hole distance but science is not that exact, so the only way to get best combination of the two for best sharpness is by experimentation. Since no pinholes are exactly the same, even in production, each is going to need some fiddling. At the end of the day there is a point when return on time invested is no longer worth it and several other factors play into pinhole projection quality on any given day anyways. Sticking to well established formulas to get the right combination is typically sufficient.
