Shouldn't the outlined text be written in Past Perfect?

The author could use past perfect here. But in the first part, not the second. And because the order of the events in the plot and in the sentence is the same, past perfect is not needed. 1. First they didn't see the person 2. Next they reached the hall 3. After that they could see the person

It's a stylistic choice.

No, this is a very normal and correct way to narrate.

The past perfect would be possible here but it's not required. If there's some reason to clearly indicate that one action happened first and then (possibly at some remove of time) another action happened, the past perfect is useful. Otherwise we usually use the simple past to indicate order of events. Compare: "I didn't start university until (after) I had turned 18." --> I turned 18 and at some point thereafter I started university. "I didn't start university until I turned 18." --> I turned 18 and then (more or less immediately) started university.

Funny, i'm rewatching the movie right now and i'm paused at that exact scene I would actually say that either way is fine, but "had reached" is better

When using past perfect it is important to remember “why is it important that something happens before the other “ Not only the usage of past perfect isn’t important in the highlighted sentence but it is also wrong since both actions happened at the same time.

I'm not certain either way (both sound fine to me), but i feel like this is one of those situations where people would use "they'd" and the "d" slowly disappeared from speech. Might be wrong, just a theory.

What you're missing is that "as though he had reached a decision" is set between two commas, meaning it's an appositive (it's a noun phrase acting as a modifier on a different noun phrase), which means that its tense is irrelevant to the rest of the sentence. It can essentially be replaced by an adjective. The base sentence is "Then, he hurried off." Which is clearly simple past.

It's perfectly fine as it is.

The difference, if there is one, is so subtle that it wouldn’t matter to a majority of English speakers. As others mentioned before, it’s mostly comes down to writing style. Maybe there’s a trend to drop unnecessary auxiliaries. Good question, it has gotten me thinking.

I think it’s ok.

"They hadn't seen another person..."

This is where style comes into play. The "camera" is following the story just behind the time they happen and is telling the events in a properly chronological order after it happens. I have read many books that speak purely in present tense, where things are being told at the moment they are happening. "He leans in towards me, and I feel my eyes sting from the old rum left on his tongue. I scoot myself back on my chair, slowly enough for him not to see me writhe, and my breath trembles as my nose grasps for any sort of fresh air." It's really all about how closely behind the reader is intended to be as the story progresses. If there happens to be a flashback, for example, then a proper change in tenses can be read. It is what gives many authors their own style, just like directors in their films.

While it's an event that happened in the past (from Harry's point of view), he's experiencing it in an immersive "first-person" point of view, so it's as if the events are happening in real time. Due to that, I think it actually makes more sense the way it's written, but either would be fine.

Yeah technically it should be but their shifting the tense to note the passage of time. Because it's covering what happens in the future (nothing happens until)

No, the text is correct.

Seems fine to me

You could use it, but the meaning and the sequence of events are clear even in the simple past, so it's not required that it be used. The past perfect is one tool that can help sequence past events, but it's not the only tool and it's not required that you use it every place that you could.

I think consistency of tense is what matters here. She would be completely wrong to use present tense, for instance, anywhere except in a quotation here. Past tense narration is pretty standard and past perfect as others have said is really to imply a sequence that we have pretty logically laid out here. I understand what's happening and their use of "until" here further clarifies the sequence.

Pro-tip: JK Rowling is actually a pretty technically unskilled writer and may not be the best resource for learning English.

No, it's correct. Think about the order of events. First, they didn't see anyone. Then, they reached the entrance hall. If anything it would be first sentence that could be past perfect, as in *They hadn't seen anyone before they reached the entrance hall*, but using past perfect isn't strictly necessary if you have other clues in the sentence/paragraph to indicate the order of events.

The incidents in text are happening consecutively so no need for past perfect

Both are fine

The whole sentence is put together poorly

No

Yes, it should. It's possible that this is a more flexible rule in UK English, but it's also possible JKR is just not a great writer.