he would have met her.

[keannu]

Can I refer to 1's underlined as "imaginary result"? I think definitely I can.
What about 2? Can it be called as "imaginary result" as well?

I'm asking this as so far I have thought of "would/could/might have pp" as counterfactual statement, but that doesn't seem to help to explain 2 much.

I'd like think in terms of "imaginary result". Then "imaginary result" can either be a counterfactual statement or a predictive(presumption) statement depending on its context or conditional. I mean, in 1, the counterfactual conditional "if he had come" affects its being counterfactual, while in 2, the presumption "if he came" affects its being presumption.

1. I know for sure that he didn't come to the meeting. If he had come, he would(could/might) have met her.
2. I don't know if he came to the meeting. If he came, he would(could/might) have met her.

[probus]

I don't think this involves deep questions such as conditional, counter-factual, etc. It's just that the pluperfect tense is fading away in modern usage. In both of your examples "If he had come" is correct in terms of sequence of tenses, but it sounds old-fashioned and people just don't bother to be correct anymore. Instead they tend to say "If he came" in both cases.

What I do not know is how long this trend has been going on. Probably all my life and I am 65. But maybe a lot longer. Scholars?

[5jj]

I don't think this involves deep questions such as conditional, counter-factual, etc. It's just that the pluperfect tense is fading away in modern usage. In both of your examples "If he had come" is correct in terms of sequence of tenses, but it sounds old-fashioned and people just don't bother to be correct anymore. Instead they tend to say "If he came" in both cases.

The past perfect is not fading away in counterfactual conditionals.

1. I know for sure that he didn't come to the meeting. If he had come, he would(could/might) have met her.
2. I don't know if he came to the meeting. If he came, he would(could/might) have met her.

#1 is a counterfactual conditional, and the past perfect is essential. 'He' did not come to the meeting and he did not meet 'her'.
#2 is very different. It is possible that 'he' came. If he did not come, then clearly he did not meet 'her', but if he came, then it is certain/possible that he met her.