Not that I hadn't guessed at it before, I have confirmed the hard way that you can't give your top index a name. Now, I'm not sure exactly what I've done underneath all this, but I'll go through it with you.
  I went to my top index and clicked on the "Rename page" button. I then renamed it, RErenamed Index. It turned out that doing that did not actually name the index, but moved all of its contents to a new subpage by that name. I find it understandable that one cannot rename the index, but I think it's misleading to leave the name of Rename this page and all its subpages as such. It makes it sound like you can.

  In attempt to move everything back to the top index at once, I went to 225.REnamed Index, clicked on "Rename page", and then erased the name. I would have expected it to just remove the page in between. Instead of putting everything back, though, it just changed the page name to . as in a dot. 225.
  What's even more confusing is that the subpages are double-dotted, and 225.. also exists. Yet the subpages link down from the Top Index as well as 225. and 225..
  So, how was I supposed to move everything back? Is there currently a way to do so without going through each and every subpage to change its name? I know that typing (Top Index) would just create a subpage by that name. *confuses*
  (And FYI, I have individually moved 225..Poetry back to its rightful location, 225.Poetry. WC didn't do that on its own.)