That's true. It'd make it one of those nods for people who realised that 'Cardassian sunrise' was more than a funny name for a drink.They didn't have to do much though.
They could just have Jim shout "Hey Sam" instead of "Hey Johny" and that's it.
Yes, but remember the stink the "Cardassian sunrise" made on these boards months before the release. Nothing is just "that's it" to us fans.

In my opinion, you have it right in your first post, Kegg, introducing Jim's brother in this movie complicates things too much. It's probably why the scene between the two boys was cut (or never filmed, I don't remember which).
If the story of Jim's life after the death of his father (and the loss of that positive influence) is so important, then why isn't George's? How the Kirk boys cope without their father and find their destinies would've been a story unto itself. You can't introduce George in those circumstances and then just leave him hanging (at least, not in my opinion). And, that might be a story for fan fic, or a nice TV episode, but it's not a story for the big screen.
Maybe George had run away years ago. Being the older brother, maybe he was away at a bording school. Maybe he traveled with his mother. What is certain is George has apparently become the "Chuck" of the Kirk family (vague Happy Days reference).