Is there a method to output the analog Sidechain In signal via S/PDIF out (R)? Can I use Triple-C Single as a stereo A/D (or D/A) converter?
The signal present on the Sidechain Input is under no circumstances passed to the Outputs. The Sidechain Input is only for compression control. And, the digital I/O on a Triple-C mono works exactly as the analog I/O, regarding the routing of the signal.
The actual mono output (and Left S/PDIF out) produces a compressed signal of course, depending on your settings. The direct out (and Right S/PDIF out) takes the unprocessed signal directly from the MONO input (and Left S/PDIF in).
So, unfortunately, this is impossible. Instead of a stereo signal, you would actually get the signal on the left side + the same signal copied on the right channel.