>>> If they don't have another family member eventually arriving then someone else may have to take that middle seat.
Yep, that's exactly what happened...they had to move to the middle seat and the 'stranger' got that aisle seat!!
>>>Even if you were at the end of the C line your child seat has to go somewhere and the people in the row you choose to put the child seat will move rather than (1) have the child seat between them and the aisle and/or (2) have you reach across them possibly from a standing position in the aisle to put the bottle in the child's mouth.<<
My dd is 12, so doesn't need a bottle,

, but has to fly in the window seat or she gets claustrophobic. So, we don't really mind being in the back, I was just trying to give an idea of how many were trying to save that middle seat.
>>> The purpose of preboarding is ...
... to enable the plane to board more quickly and take off sooner. The gate agents appraise the situation and guess whether the goal will be achieved by including families with small children in the preboard line. Sometimes they guess wrong because they did not realize the families would consume the same amount of time whether they boarded first or last. By really hustling themselves if they board first, business travelers can reduce although not eliminate the chance that families with small childeren will preboard ahead of them on future flights.
So very true. I think the fastest boarding and take-off I've ever had was on a PanAM flight right after 9-11. No carry-on at all was allowed. It was amazing how fast a plane can board if there are no bags to deal with. People just get on, sit down, and off you go!!!