Page "Is Complete" variable - showing what is missing?

There are a couple of things to note about this.

One is that you can't add a custom action to the nav buttons on the player controls/nav bar. 

A second is that the Page is Complete trigger doesn't "break down" completion into finite items, it's basically binary, as in the page is complete or it isn't. So there isn't a simple way to use it to identify parts of the page that aren't completed.

You could set up and track different variables for the completion of separate parts of the page, and then evaluate the status for those variables etc. But this would need to be triggered by the learner selecting maybe a Hint button on the page itself. One way to design this might be to add a "Page Status:" field that shows "incomplete/complete" with a "What have I missed?" button.

Also, there is a setting on the Publishing Profile, "Include Completion Checkmarks", which shows checkmarks on the tabs of a tabset and other similar components as the learner has looked at them. Turning this setting on might help your learns understand if they've missed some content.