Got a chance to look further at the heater control on my brother's
Accord today. The heat control knob was only moving in the center 100
degrees of arc instead of the complete 180 degrees.
I disconnected the control cable at the water valve, and all things
returned to normal. I removed the glove box door and could see the door
linkage working as designed.
I am going to replace the valve next week, unless someone out there
thinks my diagnosis is faulty.