As for hadith #1756, the answer to your claim is in the hadith itself, “…the Book of Allah and this written paper from the Prophet…”. From the Prophet pbuh true, but not wrriten by the Prophet, instead both the Book of Allah (Qur’an) and the paper both were written by the companions.
The Prophet pbuh used to send letters to the leaders of the tribes, he also sent letters to Negus, king of Abyssinia (Ethiopia); Vicegerent of Egypt, called Muqawqas; Chosroes, Emperor of Persia; Caesar, King of Rome; etc… , None of the letters was written by the Prophet pbuh himself instead he dictated the content to the scribes and they wrote down the letters. When the messenger from the Prophet pbuh deliver the letter to a king they will say “this letter is from Prophet Muhammad pbuh” and the letters will read “From Muhammad, Messenger of Allah to…” But doesn’t mean that Muhammd pbuh wrote the letter himself.
The letters sent by the Prophet pbuh and the replies from the kings are in this link
witness-pioneer.org/vil/Books/SM_tsn/ch5s2.html You can read through them cause they summarise the message of Islam. Specially the last letter which was sent to the King of ‘Oman, Jaifer, and his Brother ‘Abd Al-Jalandi, here ‘Amr bin Al-’As (the messenger from Prophet Muhammd pbuh) engaged in a dialogue with 'Abd Al-Jalandi it basically shows the main concept of Islam.