Is the formula of seeking forgiveness, "I seek forgiveness from God Almighty, other than Whom there is no god, the Living, the Sustaining, and to Whom I repent," considered an insult to the Prophet, and should one disregard the whispers of "you are mocking" and "you are allowing yourself"?

There is no offense in what the questioner has said. In the supplication, "Al-Hayy Al-Qayyum" (The Ever-Living, The Self-Subsisting) can be recited with either the nominative ( رفع ) or accusative ( نصب ) case. The former is more common and well-known. They could be an adjective for the word "Allah" (Allah's glorious name), or a substitute for the pronoun "He" (هو), or for the purpose of praise, or the predicate of a deleted subject. The questioner is advised to ignore whispering doubts (waswasa).