Just a heads-up, in case you’ve confused some terms:
Main L/R is usually the main mix, not a monitor mix.
And PAFL is simply a function for monitoring individual input or mix channels and has nothing to do with recording.
I’m not familiar with the QU in practice, but I have, of course, read the user guide.
However, I’m also having problems to finding an answer to your question there.
If you want to record a mix, even a simple stereo recording of your MainLR to a USB stick, I don’t see your four sources listed under selectable sources, but only, and logically to me, the two channels: