Thanks for the help, Jeff!
Sorry I've been on holiday since Dec 3 (had to use up some time).
I've set things the way you suggested and the physical LEDs on the OMNI are showing that it's clocking via WC.
The problem now is (sorry, and thanks!!!!), the MTRX does not appear in the PT Playback engine list. Well, it never has yet.
I see the Dante Virtual Soundcard in the list, but it's only 8 channels.
I also don't even see the OMNI HD TB Native showing up, so I must have corrupted my install of Pro Tools. I'll try reinstalling it all today, but if you have any sneaking suspicion based on what you know of how I've set this up, I'm all ears, and starting to almost wonder if I have a bad cable somewhere.
As always, much appreciated for helping!