You have solid and good process of understanding problems, very details and careful explanation.

However I feel it missed (or you didn't show) the part of how you get from your understanding to your prototype. If you had this part it will be more easier to understand why you have that prototype.

Secondly, the test plan is lacking of clear intention of what you want to test. Instead you just gave user the prototype to freely explore, without a purpose. That's why at the end you still got the question “Will users really improve IELTS speaking through this way of practicing?” This question should be your test plan.

But however I think it's still a very good start. Keep up the good work