Do you feel hapas are getting overrepresented in the soft power of east asia/south east asia.

I think it depends on what they look, if they look more asian then whatever. if more white (but you know they are mixed asian) then I could see a problem.

Do you think they will be a threat to the soft power of 100% east asian/south east asians and marginalize us in our own spaces?

Well crazy rich asians needed to have Henry golding who isn't even half chinese to play the main character so yes (maybe better than a non-asian person but the original story was written for a full asian person so idk). Elliott Rodgers was a danger and he was mixed so that is an example.

I think you need to be careful of whoever you let into your group (regardless of race), obviously non-asian are probably never going to experience what asian people do but hapas and asian people can also try to marginalise asian spaces.