I have a personal profile for a business as well as a business page. I plan to delete the former, but unfortunately that's the one that has the desired username. If I delete that profile, will the username become available, or will it be lost forever?
This is no longer true. According to this thread (as recently as 2017): https://www.facebook.com/business/help/community/question/?id=10153399860456687 the policy is that a username, once used, is never available again, even if the page that originally used it is deleted.