I liked your fault finding procedure.

So your exhaust gasket is leaking.

If you are not sure.
You could make up a soapy solution and squirt it around the area and look for bubbles whilst turning it over.
Or talcum powder to show the where about of a leak.

Could there be a hole in the muffler or leaking seam.

Try a known good muffler.

Happy is he who penetrates the mystery of things.