Google Forms – Send Different Data from Displayed Value on Select Option

Still don't have a solution for how to directly get values of unchecked checkboxes, but I was at least able to write a function that compares the correct answer to the submission.

This splits the correct answer into an array then compares each item to the submission:

function checkAnswer(answer, key) {
var kArray = key.split(', ');
var nArray = [];
for (i=0; i<kArray.length; i++) {
    if (answer.indexOf(kArray[i]) == -1) {
        nArray.push(kArray[i]);
    }
}
return nArray.join(", ");
}

Then I could call it every time I needed to report the boxes left unchecked. For example, with the doors question:

if (doors != correctDoors) {
  var doorsMessage = "Unlocked: " + checkAnswer(doors, correctDoors);
  report.push(doorsMessage);
}

At the end, I'll compile the report with:

report.join("\n"); 

And voila! The best I could do...

Ultimately, I'm still looking for how to directly get the unchecked boxes if anyone has a solution.

You might have to open up the spreadsheet where the results are stored, and make your own manual summary. I don't think that Forms automatic summary can cope with this in any way.