I was afraid I might be stuck between a rock and a hard place. We have
one really good HVAC company in town, I'll see what they think. Also
may take some infrared pictures to see if there's more leaks I don't
know about. My furnace is only 8 years old and is 80%, so canning the
whole thing and going 96% is a money hit I was not prepared for.

I wonder if having half my vents shut (to redirect most of the heat to
one end of the house) could affect things.

If this warm winter was typical I'd be tempted toward heat pump, our
electrical rates are low here but at 40 deg N latitude, heat pump is a
tough one.