Variables might help here. Create a project variable of type Number, with an initial value of zero. Add a trigger at the start of the quiz slide that Jumps to Slide if the value of the variable is less than some value; say 8. Last, scatter on-timeline-start variable increment triggers throughout the slides in a random pattern. The trigger on the quiz will jump to another slide if the variable is less than the sum of the variable when the learner runs through 80% of the slides.
Regarding checkboxes, I don't believe check boxes are possible through I'd defer to one of the SL forum heros. I think text color is the indicator of 'visited' in the TOC
