Suppose it's been a while since me and my friend graduated from high school.
We see our former classmate heading down the street. She is pretty and stuff. So I say to my friend "She could have been your wife." meaning that the way things turned out it's very unlikely that she will be his wife?
One of the problems with these 'modal perfect' constructions is that there is often a range of possible interpretations. The one intended is usually, but not always, clear to the participants in the conversation; sometimes one speaker will ask for clarification.
Let's look at Ostap's first situation: We see our former classmate heading down the street. She is pretty and stuff. So I say to my friend "She could have been your wife."
I think it is possible to interpret this as : There was a possibility in the past of your marrying her. If that had happened, she would be your wife now.
The second: "It would have been really nice to go to the movies but I'm swamped with work."?
This is possible with the idea: I regret that I was unable to go to the movies, because I am very busy at present.
'At present' is the time around now, which extends as far back as the movie-going time. A simpler example of this would be:
I didn't go to the movies this evening because I am very busy.