I also fear, that the user is not going to participate in this site any longer.
In what cases is it appropriate (in general) to turn these kinds of questions into community wiki?
How would one turn a question in a community wiki?
Community Wiki was created in order to lower the rep threshold required for editing certain questions. With the advent of suggested edits, it effectively became a lot less useful, as anyone with any reputation could propose an edit. It remained as a option that some sites used for questions that were a bit less "formal", but even that use has fallen by the wayside. Recently SE has removed the feature that would automatically turned an answer or question into a CW if it had a certain number of edits or had a large number of answers.
I'd say that the editing system tracks changes well enough that we don't have to worry about anyone not getting credit for answers and edits. In addition, we're very good on our site (yay!) about keeping an eye on the queue, so I don't anticipate anything hampering users without the proper reputation from just suggesting an edit outright.
In terms of how to transform something into a CW, anyone can change their own answer into one by checking the checkbox along the lower right border of the answer textbox:
I'd say leave this as a last resort if you are confident that a significant number of people will be contributing to an answer. The option to transform entire questions into CW status is only available to moderators. As I mentioned above, I've found that it's been abused more often than not on other sites for questions that likely deserved to be closed, so it's not likely that we will use that option on Chemistry.
(Incidentally, if you notice any answers that had been autoconverted to CW in the past by virtue of a large number of edits -- I've seen a couple around that I took care of a few months back-- flag them for a moderator to remove that status.)
Community wiki in my opinion could be very useful to extend S.E. to questions that are useful but atypic. Like the big lists see: https://tex.stackexchange.com/questions/339/latex-editors-ides
Up-vote in this case is related to the software and not to the user answer it self.
I hope to see one day some community wiki even in Chemistry S.E. Can we make a "big list" community question for structure drawing software?