How can one respond to those who consider the Qur’an and Sunnah to be invalid, based on arguments such as the weakness of hadith, the repetition of verses for personal reasons, the revelation of the Qur’an without diacritical marks, the recording of hadith long after the Prophet's time, in addition to sectarian differences and mutual excommunication?

Allah guides whom He wills and leads astray whom He wills. What you mentioned about disparaging the Book and the Sunnah is clear misguidance. Authentic hadiths are of the utmost eloquence and clarity. The repetition of some verses in the Quran is one of its merits; it is a reiteration of stories, rulings, and meanings to purify hearts. The claim that the Sunnah was recorded after two hundred years is ignorance. Its formal compilation began at the end of the first century Hijri by order of Umar ibn Abd al-Aziz, even though reliance before that was on memorization. Hadiths were written down during the time of the Prophet, peace be upon him, though not extensively. The difference in interpretations between Sunnis and Shiites is not a justification for disparaging the religion. The questioner is called to repentance and turning to Allah, and to frequently supplicate Allah to show him the truth and grant him adherence to it, and to show him falsehood and grant him avoidance of it.