1) No.
Subtopics have been discussed a couple of times, but I don't think they will ever make it into the 1.3 branch. They are on the agenda for Geeklog 2, though.
2) Geeklog does not provide a safe way to do that. You could restrict access to the entire story (and write a second story - without the link - that is only visible to those which should not be able to download the file). But that it is not a secure solution since someone could simply guess the link.
Geeklog does not provide access restrictions for files.
bye, Dirk