On a pinhole, sharpness is actually fixed by the size of the pinhole and nothing else.
The curved plane is there to even out exposure.
It simply guarantees that the pinhole to film distance is always the same. This prevents light fall off.
As for loading, my dad built me a 6x17 curved plane pinhole and loading it is quite something. Thing is you need two posts to turn the film so that it hugs the film guide right away. So you need to bend the tip of the backing paper to slide it under the posts.