> We know that the path is valid because we have a handle (in this case the file system ensures that no parent directory in the path can be unlinked or renamed).

Thank you for pointing this out. I erroneously stated that the length of the path could increase between GetFinalPathNameByHandle calls because an intermediate directory could be renamed, but actually I've only checked that the last part can be renamed (or even deleted, though it'll still linger in inaccessible state until the handle is closed).