一歩間違えたら自己嫌悪に負け、枯れ木が折れるようにくじけてしまいそうなこの心と境遇でも。
She hates herself for every mistake she makes, with her heart getting crushed like dry wood.
(She doesn't hate herself for every mistake she makes. It means due to her circumstances and state of mind(this is actually foreshadowing), she's on a delicate balance where one wrong turn and she'll find herself crushed under her feelings of self-loathing. It's the music box that sort of keeps that balance)
千波は、決して意識的ではないにしろ、そんな危ういバランスの下に自己を確立しているのだ。
She has based her existence on this mentality, whether she makes a conscious effort or not.
(thus the reason why this sentence mentions a dangerous balance that she's living under, that the translation kind of eliminated)

燃費が悪いって言いたいのお兄ちゃん！？
But consuming fuel is bad for you, Onii-chan!
(Are you trying to say I have bad fuel consumption? Presumably in reference to the term ポンコツ that he was using to describe her. Term used for run down pieces of junk as in cars, etc.)

ここでバイト代もらえるにしろ、早くても来月なんだ。即金じゃないんだよ
Even if you work here, next month is coming soon, so you won't get paid fully.
(You won't get payed until next month, since they don't pay in cash)

マスター、あなたが見ていたのは本当に笑顔だけですか。
Manager, are smiles the only thing you care about?
(more like are smiles really the only thing you were looking at. Subtly accusing him of being a naughty naughty boy)

……なんであたしだけ注意されるんだろ
......Why are you giving me a warning?
(It's saying that she's the only one who got one out of everyone who was talking. Take that aspect out and you completely miss the point of the sentence)

……核弾頭が爆発しても知らないからな
......But I don't know when the nuclear warhead will explode.
(don't come crawling to me if the nuclear warhead explodes on you)

それに、千波ちゃんって接客業に向いてると思うしね
Besides, I think Chinami is heading down the path of the service industry.
(she is suited for that type of work, not heading down the path)

核弾頭の千波は、冷戦というと言葉は悪いが、みんなを仲良くさせる力を持っていると。
Something about Chinami, as a nuclear warhead, would be bad in the cold war, but has the ability to make everyone get along.
(not "would be bad in a cold war". That is the effect that nuclear warheads had in the coldwar as a form of deterrence. The bad part is him saying that it's not a pleasant analogy, but it works.)

ま、異論はねえな。聞き込みのときも、千波ちゃんが相手だと向こうは警戒しねえし
Seems like you don't have any problems. Though when Chinami-chan is doing the information-gathering, people aren't as on-guard.
("Seems like you don't have any problems." should be "no argument there"

せっかくの長所も余りある短所に埋没しているのが我が愚妹なのだった。
My idiot-of-a-sister's strong point is too strong, and is turning into a bad point instead.
(supposed to say that she has so many bad points that it drowns out her strong point)

有名じゃない土産品というのは本当なんだろう。
Is it really a not-so-well-known souvenir like he says?
(This is not a question. He is affirming the fact that it's not well known, as followed by the previous statement of how they never saw one before)

音が鳴らないというのもあったし、もの自体が古くて脆くなっていたのかもしれない。
The reason I didn't hear her doing it was probably because it's old and brittle.
(More like 'it is old enough to the point that it doesn't make sound after all. It stands to reason that it's probably old and brittle as well" Also she technically didn't hit it she just shook it but whatever, hitting it is cuter.)

だってお兄ちゃんと同じで千波の寝坊も習慣になってるんだもん！
Then why doesn't Onii-chan get a habit of sleeping-in like Chinami!
(She's saying that she formed a habit of sleeping in, just like Yoh formed a habit of waking up at 6)

さすがに市販のようなオルゴールを作るとしたら、曲目は選べない。シリンダーの加工が必要になるからだ。
The tune wouldn't come remotely close to the commercial ones in shops, as the cylinder would need processing.
("If you tried to make the same type as the one you could find in commercial shops, you wouldn't be able to make your own tune because to do so, you would need to process the cylinder" which is of course not within the means of a gradeschool kid)

なら、修理代くらいは目をつむるだろう。
Then maybe it's due to the repair costs?
("The repair costs shouldn't be an issue." As followed by the previous statement talking about how it's an important symbol of the club)

元顧問の自作なのは本当なんだろう。
Is it really true that the former adviser made this himself?
(Again, affirming that the former adviser did make it himself, since it's not in any shops. Not a question. The last time this mistake happened also ended with なんだろう.  Translator doesn't know what that means?)


……彼はわざと修理をしていなかったのかい？
......Maybe he purposefully didn't fix it?
(This is more like him responding to Yoh's accusation so something like "you're saying he purposefully didn't fix it?" would be better)

たぶんそうじゃないかと思うんです
That's probably not the case.
(That's what I was thinking/That's probably the case. This is not supposed to be negative.)

教師に専念、という言葉に引っかかりを覚える。
I notice a difference in tone when the manager said "teaching".
("The phrase 'devoted to teaching' caught my attention." There's something about what that implies that made him curious and caught his attention)

なんとも判断しようがない情報だ。
There's nothing backing up this gossip.
("There's not enough information to make a call one way or the other")

過保護なやつはみんなそう言うんだよ
But everyone says you are.
("That's what they all say/That's what all the overprotective ones say." Similar to the idea of all guilty people say they're innocent, all drunk people say they're not drunk, etc.)

宇宙人がＤＶＤ観てるのかもっ
Maybe he's watching an alien DVD.
(Maybe the alien is watching a DVD)




Lazy

展望台に霞でも生えてるだろ
But the mist at the lookout is still living, isn't it?
仙人になるのも千波には荷が重いよ！？
Becoming a mountain spirit is too much too ask of Chinami!
(There's an old saying that 仙人 have reached a level of enlightment where they can persist on mist alone. Yoh is saying there's mist at the lookout and Chinami is like I'm not a 仙人. It doesn't translate very well but this is just lazy and awkward.)

もう冥土に旅立ってくれ
Please go to Hell.
(I don't expect you to translate every pun joke, but when the dialogue after the joke is people talking about it being a joke, you have to do SOMETHING there. You can't have dialogue reference a joke that's not there.)

おまえはどこぞのレスラーかっ
Are you some wrestler from somewhere?
(Ditto on this one. The wrestler joke comes from the DA! ending, which isn't present in English. Now you just have a nonsensical joke. Do something about it you lazy bones)


Tweaks

言われてみると、見覚えがあるような……
Now that you mention it, I do remember it somewhat....
(change "I do remember it somewhat" to "it does look familiar" Obviously he remembers it even before saying that line.)

メテオライトって、霊験あらたかなんだよね
Meteorites help miracles come true, right?
(would probably be better as have miraculous effects)

僕は塾に通ってないし、時間もあるからね
Yeah. I don't go coaching, so I have lots of free time.
(who uses the term coaching for cram school, etc.? Is that a European thing or something?)

できるなんて話は聞いたことがないよ
I never knew music boxes could open.
(he's talking about this specific music box not in general)

聞いてみたら、本物だそうだよ。言い切れる理由とか、ペンダントとオルゴールの関係は話してくれなかったけど
I asked, and it's apparently real. He would tell me why he was so sure, and relationship the pendant has with the orgel.
(just a typo I'm assuming, he forgot to put the n't on would. It's supposed to be negative)