OK, this may be the final chapter on this saga...
The 2-ton vs. 3-ton issue was freaking me out. So I spent $50 on HVAC-Calc Residential to run a Manual J on my own. Guess what? No matter how pessimistic I am with entering the data, it tells me a 2-ton cooling system is plenty adequate, as is 40,000 BTU heating. It took me all of 30 minutes to run those numbers. Why the heck don't the estimators use something like that when they visit the house? That's baffling to me.
If I'm to believe the Manual J, the original system was in fact sized correctly from Day 1, but if that's the case then the heat pump probably never worked right for whatever reason.
So, I cancelled the new furnace and told them to swap out the 3-ton system for 2-ton. They swapped the heat pump, but left the coil a 3-ton because it's too much trouble to change it out again. Does that seem OK? The system is running now and it seems to be performing very well in cooling mode, on a pretty warm day here. It will get a workout this weekend which is supposed to be very warm.
This was a learning experience for me. EEK!