Is Umar ibn al-Khattab responsible for the misguidance of the Ummah due to his saying "The Book of Allah is sufficient for us" on the Day of the Calamity of Thursday, even though the Prophet wanted to write a document by which the Ummah would not go astray thereafter?

It is a mistake to think that writing the document would have prevented the fragmentation and deviation of the Ummah. The Holy Quran is a clarification for everything, and Allah has not neglected anything in it. It is Allah's strong rope, and the Ummah will not go astray if it holds fast to it. The Ummah fragmented, and some of it went astray, due to the pursuit of whims. As for the document that the Prophet, may Allah bless him and grant him peace, wanted to write, it was to entrust the Caliphate to Abu Bakr al-Siddiq.