The master channel is always recorded, that‘s the compressed audio beeing recorded directly. I think it‘s actually a bug if you can switch it off 
…which doesn‘t have any impact, but I‘ll double check.
As for your other request, your choice of which channels to record also determines which channels are included in the compressed stereo stream, so you can exclude the „chat mic“ and eventually discard the recorded files on the Performer side. I seem to recall there used to be a „Record HD“ button which led to complaints…