I have two Microsoft accounts, but Settings just says Windows 10 is linked to your Microsoft account, but doesn't say which one. How would I even reinstall it if I had to?
You log in to the MS account. It should list the relevant PC.
If not, log on to your other MS account and look around.
For a reinstall?
Assuming this system is Activated, just reinstall and skip the license. It should activate itself later when you go online.
Seriously. It has been this way for 3+ years.