Sea is more natural unification area than two river basins. Much easier.
Never denied that, but it's also more difficult to hold together with longer borders and more influences coming in from all directions. (Europe, near east, ect) Also cultural unity is more difficult, especially with a breakdown in trade during bad times.
Rome lasted longer and was same size as Qin, so it would make more sense for Rome to Romanize the Huns/Germans/Arabs than for Han to Hanize Mongols/others.
Absolutely not, the Chinese dynasties from the earliest years had a far higher percentage of "Han" peoples than the Romans did. Basically Qin, Han, ect started off with a large Han ethnic populations already, whereas Rome had to build their own ethnic identity from a massive stew of different peoples and dynamics. Totally different, and it makes it harder to assimilate newcomers if your empire is super diverse already.
Plus the point is that after the barbarians succeeded in invading and becoming rulers, the geographical entity of "China" remained intact precisely because geographically it was easy to unify (the core at least). This is totally different from European geography.