You can't remove the lens (catch on the left of the body) unless the camera has been wound on, thus recocking the shutter. The mirror doesn't automatically return to the viewing position after exposure, this also only happens when you wind on, so it can be an easy way to tell what state the camera is in - if you can see through the viewfinder, then the lens is recocked.
You can't fit a lens to the camera unless the shutter has already been recocked (you can trip the shutter manually with the lens off the camera, I think it is detailed how in the manual)
Dependant upon the age of the back, you can only remove it once the darkslide has been inserted correctly. There is a little pip at the one end of the slide which disengages the interlock mechanism. For this to work you need to make sure the darkslide is the right way up (the symbol on the slide should match the symbol on the body). If this is the case then just pressing the release button on the left of the camera will allow the bottom of the back to swing out and the top of the back can be unhooked.
I think the darkslides for the interlock have a grey handle, whilst the older non-interlocked backs have a darkslide with a black handle. I haven't tried swapping them, but I suppose trying to use an older slide with a newer interlocked back might not trip the interlock so wouldn't let you remove the back. My camera's not to hand at the moment to experiment.
I only use the waist level finder on my SQA, so can't help about prism issues, but the waist level can be removed at anytime by depressing the catch on the right hand side of the body and sliding the finder towards the rear of the camera.