Captivate 8 - video on question slide and how to play and pause video on a question slide

Maybe.  You can try inserting the video as a Multi-Slide Synchronized Video (even though it's on a single slide) which will then mean the video timeline is locked into the slide timeline. You can drag out the length of the quiz slide and set the pause point of the quiz slide to where you want the video to pause.  Try that in a quick experimental project and see if it does what you want.  The alternate way to pause the slide would be to use a Smart Shape button as other types of interactive objects cannot be added to quiz slides.