What is the ruling regarding the doubts and satanic whispers that afflict a servant concerning the essence of Allah, Islam, and His Messenger (peace be upon him), and what is the legitimate remedy for such whispers?

The treatment for obsessive thoughts is to ignore them and not pay attention to them; they do no harm as long as you dislike them. As for negative thoughts, they are repelled by seeking Allah's help and focusing on what is beneficial. Obedience to Allah is the secret to happiness, so you must maintain prayer, increase remembrance (dhikr) and recitation of the Qur'an, and do good deeds, all while associating with righteous people and supplicating for steadfastness.