You could try to replace the matching question by a D&D interaction, since you are on CP6.1. It will allow what you want to achieve.
In the Matching default Question slide, the Clear button allows the user to reset the question. And all question slides are only evaluated after the user has clicked the Submit button.
As for the Submit question, have a look at the way all Question slides are functioning:
http://blog.lilybiri.com/question-question-slides-in-captivate
