currently, hardlink slaves link back (via pathname) to the hardlink master (but they do not have an own chunks list).

that has the issue that if the hardlink master is not extracted (because it is a partial extraction that did not include the master), we can not just create the hardlink in the filesystem (ln mastername slavename).
so we have some workaround to get at the master's chunklist somehow, because the slave item does not have an own chunklist.

maybe we can consider giving the hardlink slave items their own chunklist, so we do not need that workaround? (we would still need to keep the workaround for old archives though, until we can make sure the old method does not appear any more in old archives)

I've just read some hardlink support code of restic, they handle it like that.
Also, they seem to use (inode, device) to link to the master item (not name).