What are the supplications for the forgiveness of sins, and what are the supplications for strengthening faith and avoiding whispers, taking into account the aforementioned Hadith?

The answer is summarized in four main points:

Firstly: To increase faith and combat the whispers of Satan, it is recommended to engage in obedience to Allah and avoid disobeying Him, to associate with good people, and to keep away from innovators and disobedient people.

Secondly: Among the most important supplications for increasing and strengthening faith are: "Our Lord, do not let our hearts deviate," and "O Turner of hearts, keep my heart firm on Your religion," and "O Allah, benefit me with what You have taught me, and teach me what benefits me, and increase me in knowledge," and "O Allah, I ask You for faith that does not waver," and seeking guidance such as "O Allah, guide me and make me steadfast," and "O Allah, I ask You for guidance, piety, chastity, and self-sufficiency," and comprehensive supplications such as "O Allah, I ask You for all good," and asking for the renewal of faith: "O Allah, renew faith in our hearts."

Thirdly: To treat obsessive whispers and repel Satan, it is recommended to abandon them and turn away from them, to recite the Qur'an, to perform many righteous deeds, and to seek refuge in Allah through supplication. Among the effective supplications are: seeking refuge in Allah from Satan, asking Allah for protection from him, seeking refuge from the incitements of devils, and asking Allah to humiliate Satan, such as "Be gone, my devil."

Fourthly: The hadith (definition) mentioned in the question: "O Allah, Your forgiveness is vaster than my sins, and Your mercy is more hopeful to me than my deeds," is weak.