Does the MCx pass MIDI messages through from MIDI IN to USB MIDI?

Yes, the MC6 MKII and MC8 can pass MIDI messages through from the MIDI IN port to the USB MIDI Port.

The MC3, MC6MKII and MC8 can pass MIDI messages through from the USB MIDI Port to the DIN5 MIDI OUT port and the Omniports.

The Omniports do not send SysEx messages.