1. Counterfactual - he had the opportunity to meet her. We don't know if he did or not. (That is, if he would have met her or not.) Could - he would have the opportunity. Might - it's possible he would have - less certain.
2. I don't know if he came or not - it's not PROBABLE, it's simple possible.
The thing is, I don't walk up to you on the street, whipser "If he had come, he could have met her" and slip away, leaving you to wonder what I meant.
If we are talking and wondering how David met Christina, I might say "Oh, if he went to that party, he might have met her there. I know I saw Christina there... not sure if David was there, though."
If we are talking about how much David loves Katy Perry, you might say "Too bad he didn't go to that party.If he had gone, he could have met her. She spent the whole night just hanging out with the other guests."
Real life gives context to what say and provides the meaning.
Student or Learner