Are the doubts about God, faith, or the Quran that come to one's mind considered disbelief or a weakness of faith, and are prayers accepted with such whispers?

The whispers and satanic thoughts you experience do not invalidate your prayer, nor do they make you a disbeliever, nor do they indicate a weakness in your faith. Rather, they are proof of the soundness of your belief and your veneration of Allah. Allah has overlooked these thoughts as long as you do not act upon them or speak them, according to the saying of the Messenger of Allah, peace and blessings be upon him: "Indeed, Allah has pardoned for my Ummah whatever evil thoughts or suggestions occur to their minds, as long as they do not act upon them or speak about them." Your dislike for these whispers is a sign of the strength of your faith, for when the Companions asked the Prophet, peace and blessings be upon him, about the presence of such thoughts within themselves, he said: "That is pure faith." It is recommended to increase in the remembrance of Allah and to occupy yourself with what is beneficial. If you find something related to creed, say: "I believe in Allah."