1). It could be that the Russian and Japanese coaches have realized that the best way to guarantee success is to start early. The sport has now reached a level where you need those 3-3's to be even remotely competitive. It's easier for younger bodies to do, and so that's when they'll learn them. The US is kinda stuck in the past. They were winning a lot before the new judging system, but now it's become evident that relying on PCS is not a guarantee anymore, as that is all subjective. Tech is undeniable, and it's where the most advancement is being made (Eteri now having quad jumpers in development), and really just a simple math problem. I think the US will catch on very soon, as the message has been made very clear by the rest of the competition.
2). I think Yuna was 19. The fact is, she brought the tech to win with the 3-3's that made her invincible in her time. Mao was going with a different strategy with her 3A's, and so its not surprising they finished 1,2. Same story with Alina, she realized that backloading presents a huge advantage, even without the gratuitous component scores. So if she's physically able, why wouldn't she take advantage of the rules? It's a sport.
3). That's not what they wanted to happen...Kwan was supposed to be coronated. It's not like they "focused" more on the youngsters to coerce a win for them somehow. Things just happened that way.
The US ladies did not become worse imo. Everyone else just got better. Of course, they gave terrible performances yesterday (well, at least Karen did...I did not see Bradie because she was too early and Mirai only messed up the 3A), but if they went clean, they still could have been somewhat competitive (maybe even top 5 at best). The Canadians ladies didn't do so well last time around, and look where they are now? So it's definitely possible for the "older" ladies to come back and do well. But you also have to stay competitive on the technical side of things, rather than relying on this nebulous concepts of superior maturity and experience. You also can't say that the ISU doesn't reward older skaters when you look at the kinds of scores they hand out to Carolina for objectively flawed performances. And even if they were to be (rightfully, or not) rewarded PCS wise, they still shouldn't win if the technical components are not there. If that were to happen, then all the people who constantly say figure skating is not a real sport will have a much stronger case.