The stereo Mixes (auxes) are permanently linked, yes.
Qu-drive will record either a simple stereo program or 18 tracks, 16 individual, selectable tracks and the L/R outputs, but you must use a USB hard drive. From the USB-b port you can send 32 tracks should you use a digital stage box to expand the channel count.
Download the Qu-pad app and look at the Sb demo mode to get an idea of the unit. Also thoroughly read the on-line manual.
…and, from any mono input, it possible to use just one side of the stereo mixes as another mono mix – they are not affected by the L+R pan control, so you can effectively have 7 mono mixes if you want.