Narrative answers, how to measure "correctness"

This discussion contains a sample project that will allow you to check answers based on keyword use, but not frequency: https://community.articulate.com/discussions/articulate-storyline/free-sample-check-essay-questions-for-concepts-not-words 

It shouldn't be too difficult to modify the js to count usages.

As to learning, there really is no way to change your SL project after it is published. So the only way it might work would to programatically connect it to some sort of a database. My first thought is that even if it's possible, it is probably more work than it would be worth, but YMMV.