[The Squatter of Amon Rûdh]

I think we're starting to get silly now.

Gandalf's explanation for his mistake at Moria Gate is good enough for me. The inscription is deliberately cryptic and easily mistranslated (almost certainly written to be so), and the solution is only obvious once revealed. As he says himself: "Quite simple. Too simple for a learned lore-master in these suspicious days." This reveals how clever is the inscription: to simple and honest people, the first thing that would spring to mind would be to say "friend", but to someone of a suspicious bent, the sort of person one might want to deny entry, the obvious would be the last thing that would occur.

As for Cirith Ungol, it's clear from the way in which Faramir reveals the name that this alone should worry Frodo. He doesn't take this path because he is unaware of the danger, but because he has no other way to go: the Morannon has already been found to be too heavily guarded to make a safe point of entry and the only other way into Mordor is through Khand. This route would mean passing to the south of the Ephel Dúath and back past the Sea of Núrnen; a round trip of hundreds of miles through enemy territory. The chances of success would be slimmer than those of braving Cirith Ungol, and even success would come too late to help Mordor's enemies.