I upvoted Andrew Leach's "Option 3" answer, but I wanted to go into more detail about the comment-deletion policy that I think makes the most sense.
I think there is a difference between the "temporary Post-It" ideal of what comments on Stack Exchange sites should be and how they actually function in practice, and I don't see as much value in trying to stick to the ideal as I see in allowing certain unofficial uses of comments that seem useful.
Another issue is that it's not necessarily a good idea for the moderators to take action just because something on the site isn't being used the way it theoretically should be. Theoretically, question closure is supposed to be a temporary state [except for duplicate questions]--but that doesn't mean that moderators should be expected to delete or reopen at a whim any old closed question that is flagged by a user.
I'm not entirely clear about what counts as "answering in the comments", and for this reason, I'm not in favor of the more absolute policies against this practice. Sometimes, people will posts links (either to other SE posts or to external resources) and a bit of explanation of why the link is relevant in a comment: I think these can be useful, and therefore should not be deleted, but don't necessarily meet the criteria for a full answer (or sometimes, if the external resource is a dictionary, the question may be off-topic, and in that case making an answer post makes it harder for the question to be closed and eventually deleted). I have posted comments like this myself.
I think comments like this that contain a link along with some explanation of why the link is relevant are usually valuable and I would not be in favor of moderators deleting them as a general policy. To give an example, here are two comments that were posted by Edwin Ashworth beneath a question, In terms of <single> or <plural> noun? How to tell?:
I think this post comes close to being a duplicate. But your example seems really to be about the non-count / count usages of 'structure' (and the like); you could equally well consider 'When talking about structure/structures ...'. – Edwin Ashworth 2018-04-01 14:52:34Z
...your question is really about the ways structure/s are used, not about what people use after 'in terms of' (or 'when talking about'). / ODO gives helpful examples of abstract and concrete usages, going some way to differentiate count and mass usages. – Edwin Ashworth 2018-04-02 10:09:40Z
I think helpful information would be lost if these comments were deleted.
I also think that it is not very helpful to delete comments beneath closed questions. If a question is closed, then of course the author of the comment has no way to make an actual answer post. The closed status also indicates that the question should either be deleted (in which case the comments will automatically be deleted along with it) or improved and eventually reopened (and the comment might contain information that could help with efforts to improve the question).
In cases where an open question has unsourced comments that seem to try to answer the question, I would support a moderator (or any other user) re-posting the comment as an answer. Tchrist has done this with many of John Lawler's comments; for example:
In a comment, John Lawler wrote:
The meaning of grow that some might call a 'copula' is the sense of "growing to become" as in She grew tired. This is not the sense of grow in Babies grow quickly.
("Babies grow very quickly." In this sentence is "grow" a copula or a lexical verb?)
I definitely think this is better than just deleting a comment like this without warning.
So, to summarize, I would be in favor of moderators rejecting flags to delete:
For comments posted beneath open questions that don't contain links or point to external references, I would be against moderators deleting them without any other action, but I think it may be worthwhile to post an answer based on the comment. However, it seems inefficient to use a flag to ask a moderator to do this, since any user can post an answer based on a comment. So I think the best course of action for someone who is irritated by the existence of such comments would be to post an answer based on the comment, and then flag the comment for deletion (I suppose the "no longer needed" flag would technically be accurate, but I'm not sure how well it would work in practice. My understanding is that comments that have been flagged with this reason are just presented to moderators without any context, so the moderator viewing such a flag might not have any idea that an answer based on the comment has been posted. I have sometimes used custom flags rather than the "no longer needed" flag so that I can give an explanation of why the comment is obsolete; in at least one case, after I raised a "no longer needed" flag that was declined I raised a custom flag that resulted in the deletion of the comment.) Or rather than converting the comment to an answer themselves, some people might prefer to first ask the author of the original answer-comment to post it as an answer.