(Fun part of the character, every time she throws a bomb she rolls a d20 to see if she accidentally drops the bomb at her feet from sheer clumsiness, which was her own idea.)
You may want to talk to her about that. In a custom game where the GM controls the difficulty, they can compensate to enable fun ideas like this so it doesn't cripple the party at an inopportune moment.
Plaguestone is a preset module. One that is swiftly developing a reputation for being unusually deadly and causing many TPKs. This is usually blamed on Plaguestone being written concurrently with the rules before the math and encounter building guidelines were finalized and ending up deadlier than its designers intended.
I would think long and hard about adding anything that could cause additional difficulty.
In the same vein, you need to think carefully about where and when to add your assassin encounter because chances are high that the PCs will already be pushed to the limit by the preexisting encounters.