Gas Combustion Air Duct Re-Route Considerations
Consulting with an HVAC technician advised:

disconnect the left duct from the fireplace just below the first floor subfloor
disconnect the right duct just above the wye on the first floor
route the furnace + water heater duct to the left duct using 6&quot; round B-Vent. The technician pointed out that it's key to match the existing brand (I have Ameri-Vent) so that the turn-and-click attachments are the same.
remove the right duct and fireplace, capping each with a sheet metal &amp; silicone end cap.