Why limit yourself to infinity? With the 50 you've got lots of DOF and a very complete distance/DOF scale on the lens. That said, I think you're confusing the viewfinder with the rangefinder. The add-on thingy that you're missing is the viewfinder; you still use the rangefinder in the body.

I'd look very closely at film flatness with the back off the body--I'm guessing this is going to buckle pretty badly in the middle, especially if you leave the back loaded for any length of time.

Konical: Nope, you're doing it right. Sometimes it does require a bit of force. Especially, it is possible to get a lens "cross-mounted" if mounting quickly, where the ring locks it on, but it's not really seated in the mount correctly. This can jam the retaining ring pretty good. Sometimes wiggling the lens while twisting the ring helps. Do make sure you're turning it the right way (counterclockwise from front). This is a pretty stout mount--you'd have to use some really impressive force to do any damage.

Aurore: The retraction mechanism on the 3.5's is the one and only bit of underengineering on this system--both of mine have scary amounts of slop also. The 3.5's are the least impressive lenses for this system anyway. They're perfectly acceptable Tessars, but there are better things than Tessars. Get a 100/2.8 (Planar clone) or a 50/6.3 (Biogon clone).