Is it inevitable that a disobedient Muslim enters Hellfire, even if for a short period, to be punished for his sins, or can Allah's forgiveness and pardon prevent that from happening?

There is no known statement from scholars indicating that sins are unforgivable. Allah Almighty has informed us about Himself that He is "the Forgiver of sin," and His name is "Al-Ghaffar" (The All-Forgiving). The texts of the Qur’an and Sunnah indicate that the punishment for sins is removed from a servant by ten reasons: repentance, seeking forgiveness, righteous deeds and expiating good deeds, the prayers of believers for a believer, the good deeds done for the deceased, the intercession of the Prophet (peace be upon him) and others for those with sins on the Day of Resurrection, calamities, the trial, compression, and terror that occur in the grave, the terrors, distresses, and hardships of the Day of Resurrection, and Allah's mercy, pardon, and forgiveness without any cause from the servants.