Past Perfect with "before", and "until" - obligatory, optional, or strange?

Hi. I want to ask about the past perfect after "before". I have read about it in textbooks, but they usually say that that's how it is used without bothering to explain why. I also noticed that CJ tried to explain this in one thread here couple of years ago. However, I still cannot see any logic in it. So probably, I am not asking why it is used that way, but only if sentences b and c are also possible.

And if they are, do they carry the same meaning, or different meaning? Note: a sentences are taken from various grammar books.

However, if you can even shed light in terms of logic on this use of past perfect, indulge me, please.

A)

1a: He addressed her before I had introduced him to her.

b: He had addressed her bofore I introduced him to her.

c: He addressed her before I introduced him to her.

2a: We reached the stadium before the match had started.

b: We had reached the stadium before the match started.

c: We reached the stadium before the match started.

3a: He went out before I had finished my sentence.

b: He had gone out before I finished my sentence.

c: He went out before I finished my sentence.

4a: I left university before I had finished the course I was doing.

b: I had left uinversity before I finished the course I was doing.

c: I left university before I finished the course I was doing.

5a: She sacked him before he had a chance to explain his behaviour.

b: She had sacked him before he had a chance to explain his behaviour.

c: She sacked him before he had a chance to explain his behaviour.

(In one grammar book sentences 1 and 2 were split into two. He had addressed her. I had not introduced him to her before. We reached the stadium. The match had not started. Which is fine. I understand that. But in my view, it still doesn't justify the use of past perfect after "before" when I put the two sentences together. I really can't see any logic in it whatsoever In my language we only have two tenses, the past, present and future. On top of that we do not have simple and continuous forms. So you can probably imagine, how confusing the English tense system can be, and how difficult it can be to understand where the past perfect is obligatory and where it is optional. Basically, I can only see it obligatory in third conditional sentences, and with the preposition when in time clauses to avoid confusion. For example,When she arrived we had dinner. When she arrived we had had dinner.)

B)

This curiosity seems to appear also with the preposition "until". Again, a sentences are taken from a grammar book. Are b and c sentences also possible, and if yes, do they have the same or different meaning?

6a: The teacher waited until all had finished.

b: The teacher had waited until all finished.

c: The teacher waited until all finished.

7a: I didn't know what he was like until I had met him.

b: I hadn't known what he was like until I met him.

c: I didn't know what he was like until I met him.

Thank you very much for your comments. I am trying to write it down in a dumbed down form so it easy to use for as many students as possible.