Re: Why was O'Brien not an officer?
I think the TNG wardrobe issue was simply a matter of not wasting the costume budget on such a small character.
I think the intention in TNG was that O'Brien had attened the academy and was a comissioned officer.
If they wanted him as an NCO in TNG they could have just left the rank pips off.
On the continent of wild endeavour in the mountains of solace and solitude there stood the citadel of the time lords, the oldest and most mighty race in the universe looking down on the galaxies below sworn never to interfere only to watch.