1. Can the R-10 read that film (100D) properly so that the camera will correctly expose the film?
I haven't tried that one so I am not sure. Consider taking a light reading to check it out. Maybe someone else knows.
2. Should I insert the filter key into the slot that is on top of the camera?
If this is a daylight balanced film, you should definitely insert the filter key to push the No. 85 (orange) filter out of the way. However, if you shoot any interior scenes with 3200k tungsten lights, you will need to mount a No. 80B (blue) filter.
3. Has anyone used the lens in Macro Mode? If so I'm just curious as to how clear images are in this mode.
Not recently, but I have in years past. It is excellent, but you must focus very carefully. What is nice about the R10 is that you can play around with the macro shifting sleeve to customize the setting.
If you want to zoom with macro, you will need to use a diopter lens attachment instead of shifting to macro mode, but these degrade image somewhat and if you don't need to zoom, stick with the built-in mode.
4. Also wondering if anyone has any tips for shooting in single frame mode. Will the camera still auto expose?
The camera will still auto expose, but there is a danger of slight exposure shifts between frames. Even a slight shift will show as flickering in the final product. You are much better off taking an initial light reading through the lens, and shifting to manual. Consider gaffer taping the exposure control to prevent shifts.
