Thanks for your detailed feedback, Jeff. I'm glad that my tips helped you.
Regarding your new questions:
Of course, if the XD can send MIDI clock, you could try setting the S770 to "CLOCK:EXTERNAL" and "TRANSMIT CLOCK:OFF" in the MIDI settings. (It is best to save these settings as an additional User MIDI template with a new/changed name.)
If you want to stick with the previous settings (S770 uses its internal MIDI clock and transmit it via MIDI Out) you should be able to use a MIDI merge box to connect an additional external MIDI controller, such as the Arturio Keystep, to the MIDI IN of the XD. Incidentally, "MIDI (Beat) Clock" is one of the so-called "Realtime messages" (like the "Start", "Continue" and "Stop" messages) and is therefore not tied to a specific MIDI channel.
But couldn't you just use the internal S770 keyboard to control the XD? For example, if you start from the Preset MIDI template "KBD & Style", the RIGHT section will be treated as "UPPER" part and transmitted via MIDI channel 1 (even if both RIGHT parts are switched OFF on the S770).
I hope my thoughts help you!
Best regards,
Chris