I think part of the problem of boarding the wrong plane might be that GAs must be under incredible pressure to get the plane turned around as quickly as possible.
I do agree about the C2 gates in Seattle. I always look at the ala cart by the airplane closely to make sure it has the name of the city I'm going to.
We flew first class from LAS to SEA last Sunday on flight 85, which continues on to ANC. There were four people in first who wanted the same two seats. It turned out that the people in the seats were continuing on and had coach for the first leg and those first class seats for the SEA-ANC leg.
I'd never flown out of LAS on the Sunday the of a busy weekend before and could not believe the HP checkin lines, which extended outside the terminal. The two long security lines extended well past the escalators. Alaska's web checkin and the first class security line sure saved a huge bundle of time.