I believe Csound processes samples internally at 64bit. When the sound is output from Csound, the audio module takes care of the bit depth. So Reaper will set the bit depth before outputting sound. Good question about the sampling rate. I never really gave it much throught, but I think they actually run independently of each other. The sample rate you set for Csound will be used by Csound, and likewise the one you set for Reaper will be used by Reaper. Cabbage can run at a different sampling rate than the host. If not, you would hear repitching when you changed the sampling rate.
 when working with sound files I believe things are different. I don't think Csound does any realtime resampling of sound files.