Probably not.
Mechanically, no- those are special options allowing those native outsider groups to choose such traits. It isn't available for all outsiders in general.
Thematically... becoming an outsider is usually not a 'pleasant' process. Many different types of outsiders require centuries of metamorphesis, and often that has accompanying pain and suffering during the process. The time spans adn drastic changes to their beings basically means that their lives as humanoids is long behind them and mostly wiped away. And that is assuming they were humanoids, let alone humans (there are countless worlds out there, all of which contribute to the great beyond).
So the scant 20-100 years they spent on the mortal plane hardly matters as mucha as what they picked up in the great beyond.
You might be able to justify it with the options such as rituals where a mortal directly becomes a demon- that transition is often less traumatic to their original sense of self, and as such they can more easily hold onto their heritage.
Ah, I thought you brought that up as evidence, rather than as the main topic.
I would imagine they would be able to. Ethnicity doesn't appear to be a direct raciail trait for humans, so it is just something kind of tacked on. So if you are just looking for regular traits used by ethnicities, then sure- the native outsiders with those racial traits count as human, and can pick human options.
And really, if you are just talking about the 'pick 2' traits, then you could probably grab any of that with any race and the adopted trait.
Not from a rules perspective. You can certainly be a Kellid Aasimar, but you are not a kellid for the purposes of the feats and traits from inner sea races. Adopted can get you access to the traits (unless you are playing PFS in which case you are out of luck), I don't know of a way of getting access to the feats.
(outside of GM fiat of course).
The ethnicity traits are all Regional Traits anyhow, aren't they?
Nope, they are definitely race traits.