Well, you made me weep, dear mystery author. This is the ending Severus should have had, and who would have thought it would be Mrs Figg to provide it.
Everything was just marvellous--the backstory, the characterizations, the inventive magic (the clock!), and Dumbledore's machinations--all perfect.
You'll forgive me if I borrow this as my new head-cannon for Severus Snape and Arabella Figg.