Well, the Control Room settings are sort of irrelevant to what you choose to export. The output or group tab would have been more directly relevant so to speak.
I don’t think you have to use your default main output. As I mentioned earlier, I separate the two conceptually and only route to whatever mix format is required. I would never route a stereo mix into a 5.1 mix-master output if you know what I mean. My stereo mix would always terminate in something that is stereo, in the mixer. I would then maybe choose to tap that stereo channel (output in my case) in/from Control Room as a source and route it to whatever monitor output I choose, which could be surround, stereo or mono.
I mean, I think that makes sense because there’s never ever any confusion about what you’re exporting. If you need to export a stereo mix it’ll always reside in a stereo path and never (part of) a surround path. Maybe I’m overthinking it or missing something.