have been is normally used in unreal or imaginary situations in the past The tense isn't that important with the modal verbs, the degree of doubt is more important in their choice. Thus I don't think you can replace the have been versions here without removing a large part of the impossibility/improbability/imaginary of the situation. htm Also check the discussion here: "Would be" or "would have been" in past?
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how do you know when to use would, could , might and would have, could have, might have in the past?This is a very, very big question. In short, you need to continue your studies of English, and all of these issues will be cleared up for you one day! There are multiple uses for each of these combinations, and it is not really possible to answer in a single post.