Regarding X-Card, people are allowed to be willing and then change their mind. The reason their decision to change their mind is not questioned is that it's none of our business - we have no idea of their background. It sounds like you are exploring conceptual edge cases more than the reality.

Regarding affects at the table, check out Three Black Halflings podcast, who have a terrific episode on voices - from three trained actors.

Regarding appropriation and representation check out Daniel Kwan (Asians Represent podcast, covered Oriental Adventures and Kara-tur) and James Mendez Hoddes (blog, cultural consultant). They should help you better understand the power disparity in crossing cultural boundaries.