Can I use Triple-C Single as a stereo A/D (or D/A) converter?
No, unfortunately you can't do that.
The signal present on the Sidechain Input is under no circumstances passed to the Outputs. The Sidechain Input is only for compression control. Furthmore, the digital I/O on a Triple-C mono works exactly as the analog I/O, in terms of 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 what you ask is not possible. Instead of a stereo signal, you would actually get 2 copies of the left channe