Tim Hunt

I think you can get some what to what you want without writing code, just using the 'Overall feedback' in the quiz settings. However, I don't think that goes far enough for you.

There is no type of plug-in that will let you add to the quiz review page - at least not specifically.

There are two types of plug-in that might solve your problem. You could create a block plug-in, that it designed to be added to either the quiz/review.php or quiz/view.php pages (or both). That could then display the information you want.

The other option is possible if you are using a custom theme. Output in Moodle is generated by things called renderers, which are a bit like templates, and a theme can override a renderer to change the output. (Well, that is how it is supposed to work. Not all parts of Moodle have been updated to use renderers, but the student-visible parts of the quiz have.)

If it is not too late, you might want to rename that. There is a strong naming convention for Moodle git repos, and according to the naming convention, that should be

https://github.com/marcusgreen/moodle-qtype_gapfill

Doens't really matter, but consistency is good.

This will not work if even two guests try to do it simultaneously.

If you edit the quiz in almost any way, the previews will be deleted.

MDL-29934

The grade history ones are really nothing to worry about. We intentionally keep the history when the original thing is deleted, I believe.

The question one is odd. It reminds me of MDL-27083. I am pretty sure none of this will prevent you from upgrading.