I think Twi has to find the orange'd animals first before reversing the spell.
If the mirror only creates shallow reflections of yourself, I wonder what it would do with some of the other ponies... on second thought, yeah, rock the entrance. Never speak of it again. Fanfics will ensue anyway.
But oh my did I ever laugh when the test turned out to be "Watching Paint Dry". I laughed so hard at that. This was a good episode.
A secret compartment in the library that help the legend book needed... exactly who lived-in/built this library?