Yes, in theory. My hunch is yes also in practice.

I've personally experienced both motherboard damage from overpower in a USB device. And motherboard damage via lightning coming in via the cable connection in a cable modem through the Ethernet.

It should be noted that I'm running under the assumption that you are talking about a misbehaving device or power surge causing damage. If not please clarify.

Yes, in theory. My hunch is yes also in practice.

I've personally experienced both motherboard damage from back power from a misbehaving USB device.

Power where power is not expected can definitely cause issues.

It should be noted that I don't entirely understand how you have everything hooked up. But if power is going into the computer backwards I would consider that cause for concern.