Well, I’m not 100% sure I follow what you’re asking, but I use the OT cue outs as sends a lot. It’s not crazy flexible, but it works.
When you cue a track, it sends stereo out both cue outs – it sounds like you want to send mono. I’m not sure you can send mono, but nothing prevents you from only connecting one of the cue outs (left or right) to whatever you’re sending it to… That’s not exactly “mono”, as it will be the left or right channel, and not left+right summed.
For the return part of the question, maybe you can use a Thru machine where you receive a single channel (mono) and turn it into stereo. See thru machines in the manual, specifically the difference between “A B” and “A+B” (or “C D” vs. “C+D”.
I think this thread touches on some of what you’re asking: