With a closed pvc intake- exuast system, chimney draw is never an issue
with a return on the furnace. But even with return closed I dought that
issue would be true as it only pulls more from upstairs.

My cool basement air unless dehumidified for me is more uncomfortable
and harder on my AC since my AC is oversized , leaving me with high
humidity issues all summer. So I keep a dehumidifier on in the basement
at a cost of 3-4$ a month in summer.