Criteria for merging duplicates?

Question

Here are some questions regarding merging of duplicate questions:

1. What happens in the long run to questions that are marked as duplicates? Will they be deleted? When? Or will they forever stay available but marked as duplicates?
2. What is the decision criteria for merging the answers of a question, and possibly also the question, that has been marked as a duplicate? When is such a decision made? Who makes the decision, or what is the decision process?
3. How is the actual merge done? I mean for instance when the answers of one of the questions don't fit the actual wording of the other question, are the answers edited to better fit the merged result? By who? The merging actor or the answerers?

Some ranting:
I've been thinking about this several times before, but it was this specific duplicate that triggered turning my wonderings into this Meta post; Fearing that, being a duplicate question, the answers, that are good and valid, would be thrown away, I was thinking of making a suggestion here at Meta for the duplicate to be merged with the old question. You might argue that all the answers of both the duplicate and the old question are quite similar and thus redundant and that there is little extra value in keeping all the answers, but I see a value in having several answers stating just about the same thing: (a) It provides different views and ways of explaining, (b) it adds credability that the answers are correct, and (c) I might find answers from some users more reliable than from others but this "reliability selection" might be different for someone else why this is a reason to keep them all. Further the answerers of the duplicate might have chosen to answer the old question, had they only known about it. But perhaps I need not fear the removal of duplicate questions and their answers?

Answer 1

I have a feeling that these answers might change in the future due to the fact that duplicates are more tightly linked than they used to be, but for now:

1. Nothing. Duplicates should never be deleted unless there is something else at issue.

2. Questions are generally only merged when we get a cross-post and then later the original is migrated here, and both have good answers. If the questions are by different users and have very different but excellent answers, and we feel that the information belongs in one place, we will occasionally merge those questions as well. I am not sure that this example fits, but feel free to explain to me how the answers complement each other :)

3. If answers need to be edited after a merge that's a sign it wasn't a good merge, but there are exceptions, and in those cases there's no specific answer. IIRC I've asked people to update their own answers after a merge, and I've also done it myself.