Had one professional look at it so far. He seemed to know his stuff. He showed that control valve on outdoor rooftop unit was apparently not able to distinguish between when it was going into heat vs. AC mode: e.g., we had heat on, and outdoor unit was blowing warm. He believed that unit had been incorrectly serviced previously; but due to the age of the unit, feels that it would be most cost effective in the long run to replace not just the control valve and any ancillary equipment, but entire outdoor unit [with a Trane model (1.5 ton, 10 SEER)], and replace thermostat.
I sounds sensible to me; the current unit is clearly not functioning properly. But this is a big job; would gladly listen to any/all advice.
BTW, what is the reputation of Trane (a division of American Standard) heat pumps like?