▼駅前のロータリーを少し外れると人混みがなくなり……住宅が増えてくる。

Amusingly this still confuses me while sober.  Station's rotary -> ???? -> and the crowd of people dies.  Number of residential houses increases?
I want to say it says "A short distance from the station's rotary the crowd of people dies off and the number of houses increases" but I think that's wrong.

"As you get further away from the rotary in front of the station, the number of people lessen (人混みがなくなり)... the number of residences increase."

▼私が杉ノ宮駅に来たのは、そんなCDの専門店を訪れるためだ……。

Seph's translation said alternative music cds.  Not sure where he got that as I just see that she came to a cd shop.  Also don't know what the tameda at the end is.  Is the tame signifying "concerning?" as in she's come here concerning the cd shop?

Sephs was joking. It's just a specialty CD shop. ため means sake in this context, so ending it with ため means that "I came to Sugi no Miya's station in order to visit the specialty CD store."

▼それと、不意に頭の上に何かが当たった様な……。

I think she's saying something struck the top of her head (okay I actually know this is right after the following sentences so you can skip this)

▼ちょ待て、なんだあれ……？

The gist is "what's that?" but I don't get what before the comma means.  The sephs translation says it means "wait" but yeah.

"Wait, what was that?" would be accurate yeah.

▼金網を乗り越えた屋上の縁で人がもみ合っている。あのままじゃそのまま転落してしまう。

They're climbing over the wire meshes connecting the rooftops and struggling.  That as it is then as it is now they will fall (してしまう).  That last sentence is gibberish all I know is it's related to falling somehow.  Help with that.

There were people (人) struggling with each other (もみ合っている).  金網を乗り越えた = past tense, so "Climbed over the fence" 屋上の縁 = literally the roof's edge, so like, the ledge! Basically, she saw them climb over the mesh and they're not struggling on the ledge.

Last part literally is "As it is right now (あのまま), if they stay like that [and don't change] (そのまま) then they will fall (転落してしまう。) "

▼何をしているか知らないけど……止めないと……あんな危険なまねなんか……。

What "???" strange but... won't stop... danger... I'm thinking she's saying it's a strange reaction but she won't stop cause she feels danger but really that's just me trying to assign something logical.

何をしているか知らないけど = I don't know what they're doing. If I don't stop [it] (止めないと)...  dangerous behavior like that....  (あんな危険なまねなんか……。) \

The と in 止めないと　is used as a conditional,  signifying that IF she doesn't stop it... and then the next sentence just says that the behavior is dangerous, so it's IMPLYING BAD THINGS WOULD HAPPEN IF SHE DOESN'T STOP IT.

由岐▼「何やってるのっっもう！」
▼階段は外か？

This seems easy but for some reason I can't read it.  Like she's saying "What?  blahbalghag Already!"  Like I think she's yelling at the elevator so "work already" would make sense but yeah I don't know how you get there.
Then I think she's taking the stairs outside.

もう is not just already, it's also used to strengthen what she's saying like. She's basically saying "What the hell are you doing GOD!"

Also the sentence sentence is "Are the stairs outside?"

▼上でもみ合っていた……影の一つがそのまま空に舞う。

After the struggle... one of the silhouettes dances in the sky...?

Not after, just 上　で もみ合っていた. So they "They struggled above me..... a single shadow [implying that they're struggling so much you can't tell where one person begins or ends] continued [to struggle] and ...fluttered in the sky (...there's really no english word equivalent for that but tl;dr OH NO THEY'RE FALLING)

▼落下。 falls
▼屋上から誰かが落ちてくる。 someone falls from the rooftop
▼私は無我夢中で走り出す。 I lose myself and begin to run
▼受け止める気か―― to catch them (気 indicates like uhhh "willingness to do?" so she's idly wondering if she's running because she plans to catch the person)
▼あの高さから落ちてくる人間を―― for a man (person gender isn't specified) to fall from that height
▼いや、さすがにそれは無理―― no, as one would expect that's very unreasonable
▼でも、鍛えてるじゃん―― but, it's training isn't it? (no she's saying that she's been forged, so with the previous sentence it's like "Wow this is impossible, no wait haven't I trained for this...")
▼いやいや、そんなの関係ないって―― begrudgingly, such things have nothing to do with me
▼それ、普通に私も死ぬって―― that generally makes me as well die...? [No it's more like uhhh "Normally, in that situation I'd die."]

I kinda feel like this whole thing needs to be pasted because I'm thinking I might've screwed something up somewhere or missed something

I think she's saying like... someone falls, and she rushes to the person without thinking, cause falling from that height is absurd.  Then she says something about discipline/forging/training and how it has nothing to do with her and so she shouldn't do it because it'd kill her too?  I think I got confused somewhere in there.

▼となにやら冷静なつっこみがアタマの中を駆けめぐる。

And for some reason I'm calm as I plunge in... rush inside atoms (atama)?  What.  What's atama.

頭

....Uh.....
No.
"For some reason these serene retorts reverberated inside my head. ”

▼なんでこんなにいろいろ考えられるんだ？と思うぐらいに……、why have these various considerations? and to think about...　 (no here the と indicates that the previous stuff were her thoughts so, "I thought vaguely 'Why have these various considerations?' )
▼そう言えば……、which reminds me...
▼すごい速度で落ちてくる……と思われた影は……思いの外ゆっくりと落ちてきている……気がする。 falling and coming at a dreadful speed... thought to be a silhouette... surprisingly at ease to fall towards the ground... (is falling surprisingly slowly) I have a hunch. (I noticed)


▼これが死の直前の集中力というやつだろうか……。 this guy's death happened just before it seems
(Is this the ability to concentrate right before death? )

I think I massacred the last two lines... translated the whole group though just to be sure I was getting the gist of her internal dialoguing.

▼俗称、走馬燈ってやつですか？

popular name, revolving lantern fellow?  ... is this a name?  At least, the stuff before fellow should be, right?

やつ can be used for objects too. Think of it like... 'it'. not really bad when you say it to objects but to people reallyyyy bad. But she's thinking "Is this the popularly termed '走馬燈' thing?"