Does a Muslim fall into apostasy if he imagines insulting Allah and His Messenger, while immediately seeking forgiveness and disavowing that?

We advise you to fear Allah and not to indulge in obsessive whispers (waswasah), for waswasah is one of Satan's tricks to distract a person from the sweetness of faith and the tranquility of obedience. You must completely ignore these whispers, seek Allah's help, consistently recite the two 'Mu'awwidhatayn' (Surahs Al-Falaq and An-Nas), and frequently recite the Qur'an and engage in the remembrance of Allah, as the Prophet, peace and blessings be upon him, said: "Indeed, Satan places his snout on the heart of the Son of Adam. If he remembers Allah, Satan shrinks back. If he forgets, Satan devours his heart. That is the lurking whisperer (Al-Waswas Al-Khannas)." What you mentioned about imagining people cursing Allah Almighty is not apostasy in any way.