If you record say channel 1 on USB, then when you patch USB 1 to channel 1 in the I/O screen, you change the “cable” to that channel from the live input to the USB input.
If you only use 16 channels, you could patch the 16 USB to channel 17-32 instead. OR you could make an I/O library with Live inputs and another with USB inputs (that is what I have done on my SQ5, with USB-B 32 channels)
As for patching the TB to the ME’s:
One way could be (if you have a switched mic): I/O > Tie Lines >on the ME tab: Local > local talk > patch the TB to one of the ME “channels”
Regards
Søren / Soren