The magnifier was a challenge for me too at first. I kept pressing it down slightly with my eyeglasses. Than never happened with a Rollei TLR. Once I stopped pressing my fact to the magnifier it became much easier to use. When focusing 501 C/M I roghly frame the image, then use the magnifier to get it in focus, then flip the magnifier out to final frame the photo and expose. This is not quick, but it isn't too slow either. If I need to be quick I use a 35mm SLR or rangefinder.
I don't think you can find a better screen than the one that came OEM on the 501 C/M.
Maybe what you should be considering is using a prism onthe Hassy. I find them easier to focus with but heavy.