Will a Muslim be held accountable for the whispers of Satan concerning the Divine Essence and the authenticity of the Qur’an, and do these whispers indicate disbelief (kufr) or hypocrisy (nifaq), knowing that they began to appear after repentance and commitment?

You are a believer, not a disbeliever or a hypocrite. The presence of intrusive thoughts (waswas) is a clear sign of faith, and you will not be held accountable for them as long as you dislike them and your heart has not assented to them. As for the appearance of these thoughts after turning to Allah, it is natural, because Satan strives to mislead those who turn to Allah. Intrusive thoughts affect everyone who turns to Allah, and it is necessary to persevere with patience, cling to remembrance (dhikr) and obedience, for by doing so, Satan's plot will be thwarted. Satan does not whisper to a ruined heart filled with sins, but rather attacks the heart that has begun to flourish with faith. Continue your spiritual ascent, turn away from intrusive thoughts, seek Allah's help and rely on Him, and frequently make supplications and seek refuge (with Allah).