Smokey, just a thought on the first segment where you say:
"type like the DC9 where the Gear doors must open during retraction, thus increasing drag"
While I understand the profile drag increase with gear doors open, as mentioned in various texts and by instructors, I'm wondering if there isn't a further drag contribution due to the decrease in the surface area providing lift. Seems to me the area on the underside of the aircraft wing has decreased by a ratio of the door areas to the wing areas, plus there are the effects of the disturbed airflow under the wing around these doors. Since lift must remain near constant, and S has effectively decreased, now the aircraft has to fly at at a higher angle of attack thus producing a higher component of drag due to the aircraft's increased aoa profile drag, as well as increased induced drag.
Am I talking nonsense?