If the party needs experience before leaving Northminster, have them stumble onto The Rotten Underbelly (Dungeon Delver's Guide) while doing errands for Belton (instead of a mayor putting out bounty for rats, it is the proprietor for some inn or tavern, probably Ms Margo from the Thrifty Griffon? Or perhaps Dr Tinbellow from Tinbellow's Practice? If worse comes to pass and the players refuse Belton's help, they might have to escape the guards through the sewer?).
Also, while looking for Granny Applessed's ingredients, or while doing one of the quests in Hengitsbury, I can always drop The Serpent's Maw from DDG? (I guess that could work as it would explain the kobolds in the area...)