Have/had denotes possessing {an entity}.
I have {seen the movie six times}. I will never see it again. Unfortunately, I will not be able to erase my having {seen the movie six times} from the history of my life.
I am seeing Helen. I have {been seeing Helen} for two weeks now.
I have an ulcer. I have {been having an ulcer} for two weeks now.
I had an ulcer, which was removed last year. I have {had an ulcer which had been removed} for two years now. I have {had an ulcer} for two years now. I am glad it is gone.
Last month is May:
- I had {been seeing Helen} in May.
- I had {been seeing that movie for six times} in May.
- I had {seen that movie six times} in May.
- I had {shot six bears} in May.
I had {had seen that movie ten times} in May. In May alone, I had {seen that movie six times}. By next January, I would have {had seen that movie fifteen times}. Prior to May, I had {already seen that movie four times}.
1492 vs 1942
First, the tenses are misaligned in the following.
In 1492, Columbus had {landed in Bahamas}, but he mistook it for an island off India.
You should say,
In 1942, Columbus had {landed in Bahamas}, but he had {mistaken it for an island off India}
Past perfect denotes completion:
In 1493, Columbus was depressed because in 1942, Columbus had {landed in Bahamas}, but he had {mistaken it for an island off India}.
Let's presume the trial of a murder was taking place in 1500.
Magistrate: Why was Columbus depressed in 1493?
Witness: He had a mistake in 1492.
Magistrate: What mistake had he made?
Witness: In 1942, Columbus had {landed in Bahamas}, but he had {mistaken it for an island off India}
The trial places the narrative at 1493, requiring any events in 1492 to be placed in a completed state.
You can still say,
Magistrate: What mistake did he make yesterday?
Witness: In 1492, Columbus had landed in Bahamas. (Yesterday,) He mistook it for an island off India.