Again, I pretty much agree with you. I should add a few disclaimers:
I'm not a baseball fan, it's not popular in my country. I am literally in this for the character relationships and the team, so cannot disagree with anything you said about the other players having their chance to shine. Being a fan of Miyuki doesn't make me blinkered to the team, it's the opposite I think.
I am not totally up to date with AII because I don't read manga in English, which means buying the volumes from Japan and thus trying to save on shipping by buying them in groups rather than on their own. It means prioritising a bit, and since #17 is coming out soon I am waiting a while for that. So some of the things you've mentioned I've not looked at for myself yet and have only heard about from other people/sources. I'm not worried about spoilers though :)
My concern with the first years is from a storyline perspective. Introducing all the new characters could shake up the team dynamic, and I feel like maybe too many new characters are floating around. Though I wouldn't be sorry to see Seki (Na!) disappear forever...that's just my personal preference.
I don't generally subscribe to other people's opinions of how things should be, but I've seen a lot of comments like you mention re Okumura and Sawamura as a new battery or whatever. I would rather see it being as you said it, because that dynamic is interesting.
There's one other thing here which has occurred to me going back some way, which is the sort of parallel between Tesshin and Eijun in terms of their play. True, Eijun wasn't ace in the first year, but otherwise, there are similarities. And ultimately, Tesshin gained the respect of his team and took them to the semi finals of Koushien in his second year, not his third year. I have wondered whether that's a pointer towards the end of this, although it's also been said the mangaka hasn't chosen his ending quite yet.
I am also not of the school of thought that Kataoka or Miyuki didn't acknowledge Eijun's potential as an ace until the point we are at now. I think it's fairly clear throughout that that's not the case from either of them. It's just that he has to answer those expectations and perform. There's a big difference between the possibility and the actuality, and they're both pretty pragmatic tacticians when it comes to this kind of thing.
I know a lot of people got really angry at the finale of the Yakushi match in the Aki-taikai that Furuya got to play the last inning, like it somehow undermined Eijun, but for me it wasn't that at all. Kataoka even says it that he didn't want to show Yakushi any weakness. Knowing they were intimidated by/beaten by Furuya in previous games, putting Furuya on the mound and keeping the injured Miyuki in obviously would take attention towards Furuya as a threat and away from Miyuki not being fit. Keeping Sawamura in and putting in Ono would have highlighted that far more, and the batters in question would've had an opportunity to acclimatise to Eijun from the previous innings, creating more risk of Miyuki's obvious weakness at this point being exploited. To me this decision was the same kind of psychological play as putting Tanba in the bullpen against Akikawa even though he never was going to play in that match. Miyuki even says before the Yakushi game that when to use Furuya was going to be key...but if you are only able to play him one innings, then you surely only use him at the end if you need to to really disrupt the other team's flow. I think the real difference maybe in AII that Eijun is now a person that Miyuki etc feels they can rely on, and thus he has become an ace, rather than just being acknowledged as having potential to be one.
That would mean that in reality AII is really about the team growing around the ace and catcher, and Sawamura proving he really can now become that ace and take his team all the way.
But as I said, not totally caught up yet, so *shrug*.