From what i understand, all human are from the same species, h.o.m.o sapien, only of different races.
Races is not the same with nationalities, peoples of different race can be of the same nationality. Like my country, a multiracial country. being in the same nationality do not make the same race.
Human Ferelden and Orlais are different race and nationality. Human Orlaisian who live in Ferelden is Ferelden by nationality but not of race
That race is not the same as nationality is exactly why your first post didn't make any sense and why people brought it up as wrong. You made it sound as if Fereldans and Orlesians are different races, not different nationalities. As you yourself point out, people can share nationalities while being of different ethnicities.
Again, race is an artificial concept, not a biological one. Biologically speaking there is no such thing as different races. We use it mostly for the sake of convention now, and as a sociological distinction.
That said, in keeping with the accepted usage of race, neither Orlesian nor Ferelden refer to race, but to nationality. An Orlesian living in Ferelden remains an Orlesian by nationality, at least if we assume that the someone in question identifies as Orlesian in the first place because they are from Orlais. Orlesians and Fereldans can't be said to be different races; rather, it's that both nationalities can each be made up of persons from different races.
I believe Rivaini is a label more akin to a racial marker, which is why Vivienne can be said to be an Orlesian Rivaini, or an Orlesian national of Rivaini ethnic descent.