As a sound engineer in a live venue I share your frustration with this. I was surprised when I first got the console that it worked this way.
In reality, it doesn’t actually cause a problem, for me at least. I wait for a brief gap between tracks in the background house music between sets and switch shows then – keeping my right hand ready to adjust the master. The drop in audio is only ever a fraction of a second and occasionally there us a very brief click, but nothing the audience would ever notice. I should add I have a channel dedicated to the house music in every show and always with identical settings, and the same PA / fills are used for every show.
For stereo inputs there is the stereo image control in the preamp settings where you can choose L/L for example to drive both sides of a common (to all shows maybe) stereo input from a single mono socket.
Unfortunately, there isn’t a similar stereo image function for outputs to sum a stereo bus to a single mono output socket. If there was then the limitation could be worked around, but I suppose that requires processing and the phase coherency considering too so not as simple as it sounds to implement.
