1.
The first question is about the tense we should use after "as soon as". In a sentence such as this:
I had left when the phone rang.
you need to use the past perfect in the second clause to show which action came first and which – second. However, when you use “as soon as”, the sequence is clear and it is normally a matter of preference which one to use, so both your examples will be correct. In American English the preference would normally be past simple. The past perfect would emphasize the fact that one action was complete before the other one occurred. (an explanation given in Grammar for Teachers by Andrea DeCapua)
2.
In the second pair of examples they are both correct again. It is unnecessary to use past perfect because the time is mentioned and the sequence of events is clear. Also, the actions are described in the order in which they occurred. You can use the past perfect if you want, to emphasize that one was before the other.
3.
The third question was about the sentence
He said that the moment he first met her, he felt something special and began to keep a diary.
The actual words the man said must have been:
"The moment I first met her, I felt something special and began to keep a diary."
When you report his words and begin with “He said”, the entire phrase shifts one tense back and becomes:
He said that the moment he had first met her, he had felt something special and had begun to keep a diary.
Although this is the grammatically correct sentence, it is very common that the past simple does not become past perfect in indirect speech. When reporting, native speakers tend to make present tenses past ("I am studying" - "She said she was studying") but very often do not care to make the past tenses perfect, as grammar books always teach us we should.
That is what makes both these sentences correct: "He said that the moment he first met her, he felt something special and began to keep a diary." and “He said that the moment he had first met her, he had felt something special and had begun to keep a diary.” (have a look at the end of this page)
The past perfect tense is used when you need to express the order that two past events occurred. If the order of the past events is easy to understand from the context, using the simple past tense is clearer.
For example, "I had read the book and I went to the movie to see how the director interpreted the story." If I used the simple past tense, there might have have been some confusion about whether I read the book first, or saw the movie first.
I would use simple past for this sentence "According to the reporter, a commercial truck and an automobile crashed head on causing the gas leak..." There isn't a lot of doubt that the crash came before the leak.
"Although one driver suffered severe injuries, everyone survived the accident." I would also use simple past here because there is no confusion about when the people survived or when the severe injuries occurred.
"The fire department has succeeded in putting out fire rather quickly and there was no further spread of fire nor explosion." This is present perfect, not past perfect, and it is used to talk about events that happened in the past when it doesn't matter exactly when they happened. I think it's appropriate in this sentence because it's not important exactly when the fire was put out, the reader just wants to know that the danger is over now.
Best Answer
Past perfect tenses usually carry an implication that something has happened before or after.
If you explicitly state "before" or "after" then simple past will do the job fine. Including the have can still be done for a form of emphasis or to indicate that some time passed before the two events.
There is one situation where the use of past perfect is different and not optional: if it's not referring to an action at a specific time, which means you wouldn't be using before or after to qualify or any time expression like yesterday, etc.
In live speech most people don't ponder this very deeply and sometimes the line is blurred between these guidelines.