The DOF scales may have been designed with contact prints or small enlargements in mind.

The concern about opening the camera after winding would be more likely to affect sharpness at the centre than at the edges..

Try it on a tripod or steady, flat surface to see if the problem is with camera movement.

Is the edge sharpness more of a problem at certain apertures? If so, it may be an issue with the lens (excess spherical aberrations?).