Couple points for clarification.
6C was previously occupied by a guy who didn't show up for the flight, which could explain some of the confusion at the gate. There was only one non-rev on the flight, and he/she was not in 19D. So there was no non-rev stealing your seat or a line of non-revs shaking their head at you, those were revenue passengers.
The manifest had you in your seat, 6C.