Triggering Results slide with random question selection (Storyline 360)

HI everyone,

I'm using Storyline 360 to create a piece of e-learning. I've had an idea about a particular knowledge check, but am stuck on how to implement it.

I would like to ask my learners to identify the correct treatment for a number of possible scenarios but give them control of the sequence – the aim is that they choose the scenarios that they most frequently encounter.  I’d like to give them 8 scenarios to choose from, but with a results slide after they have chosen 4.   If they have a high score here (answered 3 out of 4 correctly) then the quiz would end.  If they fail they would need to complete with the 4 remaining scenarios.

I was thinking of creating a slide with 8 shapes (each labelled with a particular scenario).  When the learner clicks the shape a trigger takes them to the specific scenario, which will be a multiple choice quiz question).  The continue button on the feedback layer will take them back to the home slide (with the scenario they have already completed greyed out).

Is the above the most logical way of doing this?  I'm struggling with the following in particular:

1 - How do I get the results slide to show after they have answered 4 questions (or visited 4 shapes) on the slide, given that we are not specifying which 4 questions they have to answer.  Is this possible? 
2 - If I can get the above to work, how do I retrigger this in the results slide for those that failed. 

I could separate it into two quizzes of 4 scenarios each (those that fail the first have to complete the second) which I'm comfortable in producing, but I really want the learners to take the lead in determining which scenarios they review  so am wondering if my preferred option is possible.  I suspect I may need to use variables, but am not experienced in creating or amending these - I tend to rely on the Storyline inbuilt triggers/conditions.

Many thanks in advance for any input you can provide!