Mongolians were absorbed into Han, both population-wise and culturally. Those who stayed in China, that is.
I haven’t studied Qing royal blood lines and not sure of the Mongolian princesses claim.
I am sure I would have been taught at school and remembered it. (I am Chinese.)
Mea culpa.
That’s what I get for taking the word of Wiki without digging deeper. Wiki said the Empresses were mostly Mongol.
So I looked up the actual mothers of each emperor, to extent I could find them. It looks like every one was Manchu.
Secondary wives were no doubt often Mongol, but it looks like the mothers were all Manchu. Assuming the data I found is correct.
You are of course quite correct about the assimilation bit. Various steppe tribes had been raiding and conquering into China for three or four thousand years, sharing their genes with great enthusiasm.