Is contemplating the beautiful names of Allah and His greatness during prayer, in order to repel the whispers of Satan and achieve a state of humility (khushu'), Islamically permissible, given that such contemplation does not lead to imagining the divine essence?

Bringing to mind the meanings of Allah's Beautiful Names and Lofty Attributes is among the most beneficial things for the soundness of the heart and the preparation of the soul for worship, because it is the foundation of knowing Allah, and it is the foundation of every act of obedience.

This should be observed during prayer, for the worshipper is intimately conversing with his Lord. And conversing intimately with Allah means having the heart present, being humble in prayer, purifying the heart, and emptying one's inner self by remembering Him, praising Him, and reciting His Book.

Knowing Allah through His Names and Attributes is the key to the station of Ihsan (excellence) in worship: that you worship Allah as if you see Him, and if you do not see Him, then indeed He sees you. So, bringing to mind Allah's closeness to His servant in his acts of worship is among the reasons that remove heedlessness and bring about humility.

There is no contradiction between this and remembering death during prayer, for both are among the reasons for the heart's presence and humility.