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.
Yes, even your username will be free; for example:
After deleting your account any one can use that username, so you'll lose it if someone else registers it. If you want to retain it, deactivate your account rather than deleting it.
Yes, deleting a Facebook profile as well as changing a profile's username makes that username available again for use.
I've tried it myself several times. However this is applicable to personal profiles only, haven't tried that with pages.