Seller claims furnace doesn't work

Yes. In almost everything you posted.
1: By listing to what the vender stated. The venders declaration declares known problems.
2: How you opened yourself to needless liability if you reported your undeveloped findings . "there was about a 25° difference between intake and exhaust temperature heat pump was about 35 to 40 "degrees difference.
3: A Heat Pump is not a Furnace.

A heat pump operates in the air-conditioning mode about 20 times a day! Look for the post by David Anderson.