There are some Ahādīth which say that some verses of Sūrah Tawbah were found after the collection of the Qur’ān had been completed when the Prophet (sws) had passed away. Please explain.
All Ahādīth which mention the collection and compilation of the Qur'ān after the Prophet's death are against the Qur'ān itself. The Qur'ān explicitly states that it was collected and compiled in the life of the Prophet (sws). It is also known that the whole of it was recited to the Prophet (sws) by the archangel Gabriel twice in the last Ramadān before the Prophet (sws) expired. The Qur'ān says:
It is for Us to collect and recite it. So when We have recited it, follow its recital. (75:17-18)
It seems that the Ahādīth you have referred to were concocted later on to challenge the authenticity of the Qur'ān. One cannot accept them. Every single verse of the Qur'ān we have with us was revealed to the Prophet (sws) fourteen centuries ago. There is a consensus that the Qur'ān we have with us was not tampered with. It was transmitted through the consensus and through the perpetual recitation of the Companions of the Prophet (sws).