Cheers Doug! Chocolate fish is on its way...Likely to look like this:
The non-app app (!) may not be so good for summative exams, but looks like a quick revision, with short questions is enough to energise the class. Sounds really feasible! Good to create a change of pace/activity even a method 
As for being able to re-do a question - would it be a good idea to re-set the test (as a teacher) and tell the students they'll have 2 chances to respond? (I personally freak out at exams under set time and just freeze!)