Hello,
When you are talking about the default buttons Back and Next that you find on question slides, you don't have much control over them. As you probably did see they do not even appear as separate objects on the timeline. And I doubt that Javascript could realize what you want.
Try first to understand how the work flow of question slides is designed:
http://blog.lilybiri.com/question-question-slides-in-captivate
You can drag the Next button under the Clear button (see blog post), normally Next during a Quiz is not needed, since the second step (Press Y or click on the slide) will move to the next slide.
However the Back button is a more tricky button if you really want to have it. Maybe, didn't check it out, try to hide the back button to the user (make it transparent without a stroke) and have an image appear on top of that button after the first step on the question slide. The invisible Back under the image will remain active, it even has absolute priority over other objects.
Lilybiri