: Given the restrictions on who could use the Palantiri, the Dunedain may have decided that it was not a problem anyway, and that the future might provide a better opportunity to recover the palantir, which would be virtually useless in the hands of the the unauthorized.

Maybe so, but I still think that given Gondors military ability ( it conquered most of the north of middle earth) it shouldn't have been beyond them to retake Minas Ithil. They saw off the Nazgul at the Pellenor fields, when Sauron was at his height of power for the T.A, and with less forces at their disposal.