You are correct in all your statements, however...

If you are adding segments in the middle of your NA trip, you may be forced to allocate the A segments to the first six and the D to the last two. Sometimes this happens and others it doesn't.

WRT the A and D inventory, there is no D inventory on flights that do not offer business class service. Your seat will be drawn from the A inventory on those flights. Also, there is some business class service on two class flights, primarily (exclusively??) to Canada, Mexico, PR and the Carribean.