I admit that I didn't yet read past page 2.
I agree with Uhura's comment that race is what other people see you as. IMO, how you define your race will be a mixture of how others around you view you and the dominant culture that you identify with.
Imo, multiracial defines someone who is predominantly of mixed race. A person who is 1/16th white, but everything else is black or the reverse is not predominantly multiracial. However, I do realize that there are people whose looks don't reflect all of their ancestry.
Personally, I identify as mixed race or multiracial. I always have, even though now many of my family members don't identify the same way as I do. My family has changed its racial identification throughout the years.
I identify that way because I grew up in a culture and family that accepted all of those parts of me, and also because I don't look like one thing or the other. Depending on the day and the people, I could be and look like many nationalities and mixtures. I don't identify as solely black because I am not and I don't look like I am. I've also never felt this urge to try hard to fit into the black community. There have been times when people have identified me as only black, but those times are infrequent. If I went by what I was solely perceived to be to others, I wouldn't be an African American.
Race is a social construct that does affect the way we perceive ourselves. And racial identification is also defined by the people we are around. I've noticed that my racial classification by many people depends on where they've lived and who they have been exposed to.
Also, W.E.B. DuBois would not be considered black today. He would be part black. I always thought it was odd that he was considered fully black, when he was so obviously not by his looks. And from what I've noticed in life, just because you have some black blood, most white people don't consider you solely black. It depends on what you physically look like and what they've been exposed to.
All of this is to say, your race will be defined by your physical features -- not just skin color -- and culture. (Dark skin or "black" skin is not solely owned by people of African descent. There's this continent called Asia that people often overlook, and also countries like Mexico and parts of southern Italy, among others.)