Is it permissible to pray behind an imam who supplicates in the Witr prayer with: "O Allah, bless Muhammad with a prayer by which knots are untied, distresses are relieved, needs are fulfilled, and clouds are watered by his noble countenance," and "O Allah, bless Muhammad with a prayer by which You save our brothers in Gaza..."?

There is no harm in the aforementioned supplication, as it is an intercession through invoking blessings upon the Prophet (peace and blessings be upon him). Invoking blessings upon him with the authentically transmitted formulas is better. As for the phrase: "and clouds are sought for rain by means of his noble face," its conjunction is not sound. The mere supplication of the Imam with that does not prevent praying behind him.