Ian,
I believe you have a solution figured out for your desired workflow. So long as you don't have the "main" button selected for 19/20 when you do this (to avoid a feedback loop) you should be fine.
In my original note, I said that all channels including 23/24 are recorded PFL whether to the DAW or the the internal SD card. I meant this in the generic pre fader level sense, not specifically referring to the PFL buttons on the board. To avoid confusion about the "PFL" buttons themselves versus how the input signal records to the MTR (or sends signal via USB inbound to the DAW), Let me rephrase.
Channel strips 1 through 12 are recorded after the Gain, Low Cut button and onboard Compressor for that channel but before EQ and the fader.
Channel strips 13/14, 15/16, 17/18 and 19/20 are recorded after the Gain and Low Cut button but before EQ and the fader.
Channel strip 21/22 and Master Channel 23/24 are recorded at inbound line level so you are correct, the fader won't affect the signal when either of those channels is armed in the DAW. For the MTR, 21/22 will record at inbound line level when armed and 23/24 which is always "armed" in the MTR will also record at the inbound line level.
In my head, I picture the USB/MTR as analogous to an analog "tape-out" or "direct out" output jack.
Couple of notes to consider about the way the buttons on each individual/stereo grouped tracks work affect the MONITORING path of the individual tracks. Tracks that have the Main button selected will be sent to tracks 23/24. Tracks that have Sub selected will be sent to tracks 23/24 if the "MAIN" button is selected as well as to the Sub L and Sub R output jacks.
The PFL is a Pre Fader Level "solo" button which can be monitored via the Phones output jack and the Control L and Control R jacks on the board but it won't affect the sound coming to the Master output XLR jacks. This is by design because the PFL in this sense works like a "DJ's" Solo versus a Solo In Place Solo (Like we are all used to in our DAWs) Purpose of the PFL button is to check the sound level of individual tracks in more of a Front Of House scenario. So lets say you were using the Model 24 as a FOH mixer for a venue, the purpose is to be able to check the individual tracks incoming live signal, or recorded MTR track level without disrupting the signal path to the live PAs via the Master output XLR jacks. This is a side effect of the Model 16 and Model 24 being designed to be Front of House mixers that also have "some" old school Portastudio functionality. In my opinion, it's an annoying compromise in their design. Many of their nearest competitors have discrete PFL and a SIP style solo buttons.
In my own use case, I have a couple of different workflows that I use, including a crunky workflow using SUB L and R as well as both Monitor Auxs and the FX Aux to 5 of the channels of a Model 2 that I am using as a poor man's summing mixer. In my case, I have the Model 2 returning back into the board on (19/20) set to live with gain set to 0. In my case, if I only need those five channels I will use (19/20) as my print track just like you do. When I need to sum more than those 5 channels, I will use (23/24) as my print track. In all cases, just like you are doing I use 21/22 as my DAW monitoring channel for playback.
I have a video on youtube here
describing my own workflow.
I am so glad you got your workflow figured out for your needs.
Cheers,
Matt