Was there a rail across the perimeter of the room for drinks? A reasonably priced and accessible coat check? Was the event billed as standing room?
My husband plays in a regionally popular band, so I go to a lot of shows. These factors all come into play. I absolutely think drinks get priority for needing a place to be put down - because otherwise they end up on the floor.
For the most part seats are first come first serve. But etiquette wise, it is rude to use more then you need - so if a stool is for jackets, it should be for everyone in the group's jackets, not 3 people, 3 jacket stools, plus additional stools to sit on, but 1 jacket stool plus 3 stools to sit on.
In the case of the kid, he was just being greedy as 3 people don't need 4 stools. But the young woman might have been holding the stools for her friend's in the rest room or at the bar, etc - if she wasn't she was just being greedy. In the case of greedy you can always ask "hey may I have one of those stools please?" The people might say "no" (unfortunately they are allowed to be greedy, as unsavory as it may be) but often people will give one up if asked directly.