Storyline 360 Buttons and Builtin States

 I feel your pain. I've been there. This is what I have done in the past:

Used Individual variables (true/False) for each question and set the default value to False.

On each question slide I added a trigger that sets the corresponding value to True when the user selects the correct answer and clicks Submit

Slide Master-On the slide master for each circle add a trigger 

             ACTION - Change state of circle to Correct

              WHEN - Timeline Starts

              Add a CONDITION - If corresponding variable is equal to True

        This takes care of the slide loading so the circles reflect the current state of each question

Then make sure the slide properties are set correctly - Each question slide is set to "When revisiting" to "Resume saved state" so that it maintains the state of the slide and its objects if user navigates back and forth.

I would also not use the "When variable changes" trigger on Master slides because I find it unreliable. It tends not to see the changes made on other slides.  The better option is to use "When timeline starts" with conditions based on variable values.