I think it was “the [library’s] tools would [eliminate] anyone who entered the library through illicit means.” But walking in through the front or back door is fine as long as you don’t force your way in. I think you’re entirely correct about the masters, library personnel, and royalty, but I get the impression that royal branch families do not get special access. They got in because of Horenisia’s feystone, which let them into the dormatory, which appears to be a weakness. Solange maybe let them in the rest of the way after the shumils informed her of Hortensia’s arrival? They did threaten her.

The “illicit means” thing probably also applies to the masters, royalty, etc., though, as the shumils did consider Rosemyne a threat when she tried to force her way into the door that holds the copy of the G book.