Balhae not falling might have meant that the Mongol might never have come to power.
These kinds of things always award France absurdly generous amounts if land in China.
The South China thing is a bit silly unless that's occupied by some foreign power. Vietnam would never have the strength to take Guangdong alone, let alone the three provinces you have drawn out there. The Zhuang were never big enough to cause much of a nuisance east of Dai as well, so you'll have to justify this a bit more (Guangdong is/was bigger than France, Germany, Britain or even European Russia...)
Would it have the strength to take Guangdong if China is otherwise occupied by other invasions? The Goths were not nearly strong enough to defeat Rome by themselves, but they managed because Rome was weakened and spread out. In this TL, China would be weakened and spread out.
What about an alternate WWII where Japan and the Sowjet union are allies - and some southern neigbors and/or a western colonial force takes whatever remains?
On second thought, if Taiwan were to be colonised by Europeans before the 18th century you will have to butterfly away Koxinga who ejected the Dutch from their already tenuous foothold (who in turn ejected the Catholic Spanish from their foothold). I don't believe that even the most hated regime under any circumstances will not have its armed loyalists.
Balhae not falling might have meant that the Mongol might never have come to power.
I find Koguryo and Silla uniting via marriage a very cool idea actually.
My best personal is the King Uija of Baekje escaping to Yamato instead of being captured by Tang and Shilla, where he leads a continuing restoration movement. In OTL, the restoration movement collapsed because of internal conflict within the movement (because of the weak position of Buyeo Pung, the son of Uija who was considered the successor). If Uija had been able to be free, we might see Goguryeo not being destroyed because of the continuing war in Baekje, allowing either Goguryeo to survive longer. Even if Goguryeo fell, the conflict in Baekje would have weakened Shilla, and there is a chance that it might have fallen to to Baekje-Yamato. Who knows? We might actually see Japan and Korea being linked by Kingdom of Baekje.
Is it possible for parts of China or huge lands in China to get annexed by a neighboring polity like say Vietnam, Korea, Japan and even Tibet or a colonial power?
If anyone is going to take big ol' chucks of land from China, Russia's probably the most likely contender. Decades of Darkness has Russia more or less swallowing the entire northern half, part of Korea, and Tibet.