Theoden was under the spell of Saruman. Not Denethor.
Denethor was weakened by Sauron because he had seen into the pilantir and his own "greatness" was threatened. He was a pretender to the throne who was willing to surrender in the end.
He would never have kicked a terrorist out of Minis Tirith or returned a check to the king of the southern kingdoms.
Denethor was the big city mayor under the spell of Sauron...