This is quite an ambitious project, and I like what you've written so far. Guyuk Khan sounds terrible!
In relation to that scenario, what would happen to the Northern (Han) Chinese population?Genghis' conquests of China TTL last longer;
These conquests, over the course of over two decades, are even worse than OTL; the only major cities to survive in any true capacity are Chang'an/Xi'an, Kaifeng, and Luoyang, and the northernmost cities (OTL's Beijing, Xijing, Zhongjing, Yanan) are completely razed, with much of northern China down to the banks of the Yellow River converted to mostly-empty grassland.
Interesting! I do hope that all these Chinese and Arabs (and Indians?) have not died to wank Europe _further_ than OTL, in the long run: it would seem a bit gratuitous.
Minor suggested correction: in your post about the Abyssinians vs the Mongols, you suggest that the Japanese also successfully humiliated the Mongols, but a post or two earlier you said: The Mongols never bother with Japan TTL
Does Abyssinia retain sea access in this TL?
A northern border around Beijing would indicate that a. the Mongols and their heirs have established a settled agricultural society capable of withstanding early modern gunpowder empires or b. someone else got the jump on the Chinese for overrunning them. Is this Need to Know information at this point?
How it would affect the development of Standard Mandarin/Northern Chinese language, as well as its dialects?