Why didn’t Prophet Muhammad correct ’Abdullah Ibn Sa’d Ibn Abi Sarh? I’ve read some alarming things about him and his writing of the Qur’an. In several books, including al-Kafi, it says Ibn Abi Sarh changed "Mighty, Wise" to "All-Knowing, Wise" then Prophet Muhammad told him "Allow it/leave it/let it be (دعها), for indeed Allah is All-Knowing, All-Wise (fa-inn’Allah ’Aleem, Hakeem)" instead of telling him to fix it or condemning him. Is this not Prophet Muhammad taking an alteration over the revelation from Allah? Why didn’t they fix it?

I also saw (though in a Sunni source) people saying that in Qur’an 23:14, the "So blessed is Allah, the Best of Creators!" Was exclaimed by Ibn Abi Sarh, then Muhammad SAWA told him to add it.

What was the nature of these changes and why didn’t Prophet Muhammad tell him to fix it?

I would also like to know what Prophet Muhammad meant by "هو واحد هو واحد".
And if possible, is there anything in out books about the claims ’Abdullah makes in Sunni books? I.e. that the "So blessed is Allah, the Best of Creators!" was exclaimed by him, and also that on an occasion Prophet Muhammad and he sat down for a session, and Prophet Muhammad told him to write such-and-such, but ’Abdullah claimed he only wrote "write" and then Prophet Muhammad saw and told him "Write whatever you wish."

1- As for هو واحد it means whether you change the way it’s written or not, and whether you distort it or not, Allah will protect it, so don’t think you can change the Qur’an. “It’s the same” whether you distort it or not—Allah won’t allow you to do that. He’ll expose it and you won’t get away with it.

2- Another hadith explains that what is meant by هو واحد is that Jibraeel will fix what you change. So after you write it incorrectly, Jibraeel will edit it in your copy and fix it, so you won’t be able to change the Qur’an.

3- As for دعها it basically means: you want to change it عليم حكيم from عزيز حكيم? Well, Allah is aware and He is wise, and so he will protect His word and fix it. You want to distort it? Do what you want, Allah the knowing the wise will basically fix it. This is what it means it. This is one understanding of دعها

The second understanding, which is more plausible, is that the Prophet (s) told him: no, keep it عزيز حكيم not عليم حكيم even though Allah is علم حكيم. So don’t change what I tell you. Keep it as is.