For the archive option, there is a good recap here : Best practices for archiving topics
For your third request, the best way to do that would be to move the topic in a staff section where only admins would have access. You’ll have an easy access to it. Unlist and Archive would not prevent users having the direct link to access it if the topic is in a public category.
You could also delete a topic, a topic is never really deleted on Discouse, you can access all deleted topics with the link :
yourforum/?status=deleted
And for the record, you can check all unlisted topics with this link :
yourforum/?status=unlisted