Thank you so much for this wonderfully detailed review; it really made my day! And I'm glad you liked my "it would take a conspiracy of men" line. When I was plotting this thing, I got stuck at precisely what you describe: wonder that this Stone, protected by teachers who are, on Minerva's authority, rather good at magic, could be reached by three eleven-year-old first-years. I finally came up with this one.
Thanks again for putting so much time and effort in your comment. It was a lovely thing to get.