I don't think it matters what real ethnicity people are, as long as they look like the ethnicity of the character they're playing. I wouldn't be upset if they cast an Irish person to play a German part if they really looked German, would you? I mean, should they only get Jewish people to play prisoners in Holocaust films? lol
Idk who could play Mandarin...particularly cause I don't know how they can do Mandarin without totally killing off any realism that they want to build up. I'd have preferred a Justin Hammer + Spymaster + Titanium Man or something like that.