Do nothing. As long as it seems like a suitable answer we leave it alone.
This said, if the post is spammy, not related, or similar then we address it. Otherwise, we leave the questions be - no need to touch something that's not really 'broken'.
Yes, when your reputation dips below the level of a given privilege, you lose that privilege until you gain enough rep to go above that requirement level again. This is status-bydesign as part of reputation levels being tied to privileges.
If you are wondering how the question came to have an answer, you can see from the question's timeline that the answer was posted before the question was closed.
Preventing answers being posted is indeed the primary effect of closing a question. We close questions (except for duplicates) because they are not answerable within the scope of the site, that is, ...
Basically correct yes.
A moderator and/or a group of users with 4 re-open votes could re-open it, someone then could answer it and they could close it again. But that it not something we would do for a question like this. It is indeed off topic
To be fair, 590 points is just 59 upvotes. Since you have posted 44 questions and 1,577 answers on the main site so far, that doesn't seem to be an excessive number.
It seems I only lost 10 points when this user was removed, but then I've posted far fewer questions and answers than you have.