Is saying, "O Lord! O You in Whose hands are all power and might," considered a challenge to Allah, Glorified and Exalted be He, knowing that the intention is to affirm that there is no power or dominion except for Him?

There is no harm in saying: "O You in whose hand are power and might," because Allah Almighty possesses supreme power and in His hand is the dominion. He also has the attribute of the Hand, which is affirmed in the Qur’an, the Sunnah, and by scholarly consensus, such as His saying: ﴿Blessed is He in whose hand is dominion﴾ and His saying: ﴿I created with My two hands.﴾ A Muslim must submit to the texts and believe that whatever Allah has described Himself with, or His Messenger has described Him with, is true, and beware of the whisperings that cause disturbance and anxiety.