Set up all your needed channel outputs in the USB setup, just make sure you set USB channels 17/18 for L/R. All your channels will show up in your DAW, including the L/R mix on 17/18. At the same time you can record stereo 17/18 (L/R) to the Qu-drive.
Just responded to your other post, but it sounds like you already carried out the patching.
If you definitely had signal going into ST1 and showing on the Qu, and your other channels were setup/patched in the same way and did record using Direct Outs, the first place I’d recommend checking would be the Channel Direct Out Trim – this is set on a per-channel basis, and can be turned right down to -inf.