Is it considered a departure from Islam to believe that God will not guide all seven brothers for whom guidance has been supplicated, despite knowing God's absolute power, and is ghusl required for reversion?

A Muslim should have a good opinion of Allah when making supplication, and ask of Him with certainty of response, for the hearts of people are in Allah's hands; He turns them as He wills. In the Hadith: "When one of you asks, let him ask a great deal, for he is only asking his Lord," and "I am as My servant expects Me to be," and "Supplicate to Allah with certainty of response, for Allah does not answer a supplication from a heedless, inattentive heart." You should ask Allah with resolve to guide your brethren, for Allah is the Guide, the Capable, the Responder. You must rid yourself of the doubts that trouble you by frequently pondering the texts of the two revelations (Qur'an and Sunnah) which show Allah's power and that the hearts of people are in His hand, He guides whom He wills and misguides whom He wills. The filling of Paradise and Hell does not mean that the children of the same family will be separated; Allah may guide all members of some families. You should repel these whispers and occupy yourself with what benefits you.