For starters I'm happy you got a PERFECT title that fits right in! Daggers in the dark sounds fitting for a defense scenario the story is great and the mansion is great too. One thing I think it needs to be added, since this is a system agnostic scenario, I would prefer a few things 1. give us the stat blocks for the thieves 2. get some brief description about the theives (are they villagers who needed money to survive? were they rivals against another baron? or are they just like home alone bandits in a comedic silly way. ) 3 some examples of difficulty checks or any traps with dcs that you can throw in. 4. As for the theif tactics I do wished they have different plan routes and different tactics like they can split up to make the baron being paranoid or they would run away if they get spotted etc.
Overall, I love this heist that can be fun and a great way to start a campaign out of it. Do keep it up though!