You really got a cool title to work with! I am so envious... I really enjoyed the core plot and atmosphere! The open setup with the mystery target (paired with the Baron’s paranoia and Petunia’s skepticism) makes for such a fun, dynamic defense scenario.
That said, as written, it's missing a few elements to make it table-ready, IMHO: there are no stat blocks for the thieves, nor any skill check DCs provided. I understand it's system agnostic, but please give me something to work on: are these thieves cruel, efficient war-machines, or total goofballs? Also their tactic is a bit bland: it states they take the shortest path to their target, but there are no guidelines on how they react if spotted (do they flee, fight to the death, or take hostages?)
Overall, it's a creative and dynamic heist module,