George,

I can offer views on a few of the issues you raise. I have an S2 (essentially the same as your S2A but yours has stronger gears) and an SQ-A, and no, the backs are not at all interchangeable. I'm not sure where you're looking, but I haven't found lenses particularly difficult to find, except for select lens like the 80mm f/2.4. Lenses are often available on American eBay or from individual sellers or shops. As for the screen, I personally wish I could have a split-image screen on mine. Rick Oleson (google him) sells screens that will fit your S2A. As far as I know, as long as the foam supporting the screen is installed properly and you put in the new screen (and separate fresnel lens, IIRC), then you shouldn't have any problems. As for mirror slap, well, that's just the way these camera work. I don't think there's much to be done about it. Later models like my EC-TL used a split mirror to reduce mirror slap. I've never heard anything about some shutter speeds being better than others, per se, though of course some speeds are more appropriate depending on what you're photographing. If shooting a moving cat, you might opt for 1/250 or faster; when shooting a house that isn't moving, then perhaps you'd be more concerned about depth of field and let that determine the shutter speed. If you're using a tripod, you can use any shutter speed you like; the problem comes when you are not using a tripod and want to shoot at slow speeds which might cause unwanted blur in the resulting images.