How can I add multiple "correct" hot spots to a quiz slide?
Wondering that I have never noticed this thing. Hotspot does not have Selected state so may be multiple hotspots works as button set, and selecting other hotspot canceled selection of 1st hotspot.
You can use invisible buttons in freeform question. Look in the attached file.
You can use "Pick Many" from freeform by inserting a new slide or converting a present slide to freeform.
I would agree with Joanne's recommendation - as the hotspot question type assumes only one is correct. There is a video here that walks you through how to do this with the Pick many functionality. It's for Storyline 1 but the same will apply for Storyline 2.