Is there a specific act of disbelief or mockery of Allah and the prophets that Allah will not forgive, even if the servant repents with sincere repentance before the Day of Judgment or death upon disbelief, especially with the presence of obsessive disbelieving whispers?

These whisperings do not affect the soundness of faith as long as they are mere thoughts that are disliked by the heart. The proof of your dislike for them is your troubled conscience. Something similar happened to some of the Companions, and the Prophet, peace and blessings be upon him, informed them that this was clear faith.

The cure for these whisperings is to ignore them, seek help from Allah, and supplicate to Him to remove this affliction. You should also consult trustworthy doctors for treatment, because the Prophet, peace and blessings be upon him, said: "Seek treatment, for Allah, the Almighty and Glorious, has not created a disease except that He has also created a cure for it."