Thanks for the quick reply, Rob. Being able to increase the size of the page certainly does help the correction process, and I didn't know that function.
If it helps, I also found out yesterday by trailing through forums that clicking on the "Arrow" allows you to select elements that you commented on and delete them (a kind of indirect "eraser"). And some versions of the toolbar seem to now have the "undo/redo" function (see forum thread here: https://tracker.moodle.org/browse/MDL-60299?attachmentOrder=desc) but unfortunately we don't have that version of the toolbar at the university I work at...
Best,
Eleanor