I don't think that should happen. If that person knows an answer to the question, then they should post an answer. They can ask for clarification in comments. Chat also exists for extended discussion. Posting half answers in comments is already discurraged, because if the commenter's suggestion is wrong, we can't downvote it.

I would even go so far as to say talking offline like that hurts the site. SE sites are meant to be a collection of knowledge, and so questions should be useful for more than just the OP. If they talk off-site, and then self-answer, or get the person to post an answer, then that isn't terrible. But... I find that often they won't come back to do that, so it's better if communication stays on-site. Of course, we also have chat, which works just fine.

I would flag ad Needs Moderator Attention or No Longer Needed, but I'm not sure which is correct.

See also this question for a related discussion.

I don't think that should happen. If that person knows an answer to the question, then they should post an answer. They can ask for clarification in comments. Chat also exists for extended discussion. Posting half answers in comments is already discouraged, because if the commenter's suggestion is wrong, we can't downvote it.

I would even go so far as to say talking offline like that hurts the site. SE sites are meant to be a collection of knowledge, and so questions should be useful for more than just the OP. If they talk off-site, and then self-answer, or get the person to post an answer, then that isn't terrible. But... I find that often they won't come back to do that, so it's better if communication stays on-site. Of course, we also have chat, which works just fine.

I would flag as Needs Moderator Attention or No Longer Needed, but I'm not sure which is correct.

See also this question for a related discussion.