"This is not a continuing flight, so how were these seats selected prior to the 24 hour window?"

You can call the elite line and depending on who you get you can get moved into a choice seat. or hang up and call back.

"On the September 11th (yes, my will is in order) return, there were NO Coach Choice designations...every seat was available.

What is the logic that decides which flights get Coach Choice holds on the seats? "

My guess is because its an international flight on Sept 11. That there is a reluctance to fly on that date by many people, so they opened the whole plane.

I am sure they have some computer program that figures out which seats they should have based on previous load conditions over the past three-five years.