[The Squatter of Amon Rûdh]

I'm pretty sure that Shagrat was more concerned with blaming Gorbag for the entire fiasco than with presenting a decent report. We must also remember that Frodo and Sam had disguised themselves as Orcs in a country full of Orcs, and were deliberately avoiding camps and concentrations of military force; and that Gandalf, Aragorn et al were advancing on the Morannon with the sole purpose of convincing Sauron that they had the Ring. Mordor is in chaos: we see Orcs fighting amongst themselves on top of the normal disorganisation of armies on the move (I have no doubt that for this Tolkien drew heavily on his experiences as an officer in the Great War). Frankly I'm not at all surprised that two little Hobbits could get lost in the confusion, especially since they didn't want to be found.

As for the Mouth of Sauron: he's just trying to shake the confidence of the enemy, possibly even to force them to give up without a fight. Perhaps he was even hoping that someone would let something slip, but mainly it's a psychological-warfare technique that's as old as the hills.