Does the return air duct to the furnace (ours sits directly above the furnace) need to be fire-proofed? Or is a plywood/stud/drywalled return air "box" sufficient


No need to fireproof that I can see based on your description and really a return can be enclosed by anything. It just needs to be sealed.

Updated on February 16, 2018

Original Article:

HVAC: The Parts of a Residential Heating and Cooling System
By Dan Robbins