Too bad the prism doesn't come off! Then you could check the focus at both the film plane, and the viewfinder to see which one is off.

I use a small microscope for this when repairing other cameras.

Set it at 100x and focus on the ground glass. I also found that I can get very close with the built in magnifier of the waist level finder for many models, but the microscope is the final test. I have a ground glass out of an old TLR that I use at the film plane.