simple past + by 6pm

Question

This question helped me construct the next sentences and I want to make sure if they're correct:

1. By 6pm I had given him the money, and then I went home. (Would "gave" be incorrect?)

2. I gave him the money by 6pm, and then I went home. (Would "had given" be incorrect?)

3. I had given him the money by 6pm, but he wasn't pleased. (Would "gave" be incorrect?)

4. And then by 6pm I had given him the money, and went home. (Would "gave" be incorrect?)

5. He confirmed that he had given him the money by 6pm. (Would "gave" be incorrect?)

And here's an unrelated sentence:

6. "Has anyone ever told you that this sound had vanished from British English by the 19 century?" (Would just "vanished" be wrong? Why?)

Update 1:

There's been a suggestion of another question as the one that might have already addressed my question. But it doesn't seem to address it. The answer to the suggested question is broader and is about the general difference between the simple past and the past perfect tenses. My question, on the other hand, is more specific and is related to the use of both tenses together with "by + a certain time". Through reading, I've noticed that it's not always clear why either tense is used with "by + a certain past time." It seems like the placement of "by + a certain past time" phrase may affect the tense, that is if it's placed before the subject (see sentences 1 and 4) I must use the past perfect tense, whereas when it's placed at the end of the clause the past simple tense may sometimes be used, I would like to know the reason why I should prefer one tense rather than the other in those cases.

Update 2:

To address Lambie's point below this question:

I usually know what I want to say, the issue is I'm not always sure what "English tools" I should use to say it in English. When I see some written English, I try to make sense out of it, and it's been tricky for me in this particular issue.

The textbooks that I've seen seem to have muddied water for me. One in particular seems to have mislead me by stating that

The past perfect tense is used to express a past action that has already taken place up to a certain point in the past. The past perfect tense represents a "pre-past" time, since it expresses a past action in relation to a moment that is also past. This moment can be indicated by by five o'clock, by Saturday, by the 15th of December, by the end of the year, by that time etc.

The textbook illustrates it with examples:

We had translated the article by five o'clock.

By the end of the year he had learnt to speak French.

They hadn't finished their work by six o'clock.

We had shipped the goods by that time.

That plant had fulfilled its yearly plan of production by the 5th of December.

So, that article seems to say that you must use the past perfect whenever you see "by + a certain past time" since the past moment before which something happens is represented by "by + a certain past time". But when I started reading English literature and any other English texts I realized that it was not always the case and that made me confused.

Here's some examples I saw online:

Pre-production for further episodes began by July 2020, and filming resumed on September 28. Bayona completed filming for his episodes by December 23.

Source: wikipedia

As he mowed, he practiced his Voices. He finished—front, back, and sides—by three o’clock Friday afternoon, and began Saturday with two dollars and fifty cents in his jeans.

Source: It by Stephen King.

I think they should be changed into "had begun, "had completed", and "had finished" if the textbook is correct.

Answer 1

All of them are fine.

There is no particular interaction of the choice of forms with a by expression.

The difference between had given and gave is that with had given, the speaker is choosing to view the event from some later point in the past. With gave, no particular temporal viewpoint (or "story time") is set or used.

So in 2, there is no "story time" set, and the narrative simply follows the events in order - first I give him the money, and then I go home.

In 5. had given is natural, because it is viewing the giving from a particular later time - the time when confirmed it. It's still not obligatory even in this case - we don't always use the past perfect when the temporal relationships are clear. But it's more natural.

In 3, had given makes clear that the time when (I realised) he wasn't pleased was later than the giving - if his displeasure was clear at the time of giving, I'd expect he hadn't been pleased.

In 1 and 4, the events are being viewed from the time when I went home - presumably around 6 o'clock.

Edit: I had (!) missed sentence 6. In 6, had vanished is normal, because you're looking at the event from the time when somebody (possibly) told you. Again, it's not obligatory: the temporal relationships are clear, so people would often user the simpler vanished. But had vanished is the most natural option here.