Don;t get me wrong, I like Eijun a lot too. He has a lot of real positives to his character and it's not really him being loud overall that bothers me, that's just who he is. It's certain loud moments that just make me wince because he's annoying the other characters and embarrassing himself at the same time. The 偉そうに moments where he comments uninvited on things like the performances of the others, and particularly Haruichi. Although it amuses me that he uses tameguchi particularly with Miyuki, so it's not all the time. If he didn't yell those things he would probably make my top 5 characters, but there we are. He DOES have the ability to reflect on his errors and behaviours though, which is probably his real strength in becoming the Ace.
Personality wise in a lot of ways I find Furuya less annoying even though he's a lot less popular with the fandom - but I generally like them both the same because one needs the other for most of the development. Their strengths and weaknesses balance each other out so it's hard to pick one over the other (to me). It's one reason why I think it's a shame only one of them can wear the ace number as I feel they've helped to build each other.
They do all pretty much say stupid things at times, but that;s fine. It's the balance of it that's a problem for me. Raichi's constant laughing is also annoying to me. It's just those little things like that. Stuff the others say don't really annoy me. Even if they are idiot things, which from time to time they are.
Asou is a really interesting call actually, because he kind of appears almost from nowhere around the time of the new team (he's in the background before but you don't really notice). He's another background character you feel is slowly being fleshed out even to the point I've read up to. I'm glad he's also shaving off his more annoying edges. There are a few like this too - Yamaguchi, Higasa, for example. Also, one that really caught my attention is Shirasu because right at the start Eijun can't even remember his name, and yet he's such a core character to the team both in play and in terms of his personality. It's interesting that it's Shirasu who Kuramochi trusts with his fears about Miyuki being injured, because Zono (much as I like Zono) is a shade indelicate about discreet stuff. It shows how important even at that point Shirasu is to the dynamic of the team overall, even though he's also been gradually developed through his play to that point in the new team.
It would be interesting to see actual Koushien stuff. Although in another way I feel like because I'm such a detail nerd, if I understand more about it and spot the mistakes you mentioned, it might drive me nuts. xD. But the technical aspects of it are probably less important to me than the character development side of things. I'm not remotely interested in American baseball, which is all that gets aired in my country, but any attempts I've made in the past to watch Japanese stuff has been blocked by my region. If and when I go back to Japan I may try and visit the place directly...just to see it in context.
The translation aspect interests me as I used to translate manga in another community (or I should say retranslate it because the scanlations were so poor). I learned a lot then about how fast and loose people are with meanings and how it can create misconceptions. In that fandom one huge misconception was created by a scanlation group translating a Chinese version, not the Japanese raw, and even at the end of the manga series there were people who still didn't accept that that was a mistake and got angry with the mangaka for what they saw as him betraying their expectations. Tricky stuff. One reason I won't read manga in English, since I don't need to, and when it's a popular series people rush and make mistakes which then have bigger implications. I saw a whole argument over cutters and forkballs and another one over whether a particular scanlation made x character sound too pro-Furuya compared with the dialogue...so it's clearly a problem here too. I think it's fine to be aware that there can be mistakes in the translation, even if you can't read the original Japanese. The most annoying people are the ones who tell you the scanlations are right because they like them better :P even when they can't verify it for sure. As you say, Japanese is subtle and a lot is conveyed by implication, so a perfect translation isn't really possible (it's harder to write down absolute accurate English even if you totally are confident of the Japanese nuance). But when scanlation groups translate from a chinese translation or something, gaps do appear and I don't think it's a problem to question those or ask people to verify.