Just setting this gem as a bookmark :).
Would this be an accurate statement regarding RS?
The RS is made up of a relationship that we see by observing the two characters the relationship is between. The RS is the story of the relationship and how it grows and changes. Two guys just riding in a car together might not be RS, but two police becoming friends as they take down crime as partners is, or two guys becoming enemies when one arrests the other is.
does the RS throughline relationship have to exist between only two people?
Not necessarily. It seems that there can be multiple different relationships in a story, each of which is part of the RS throughline:
I also believe that there can be things like a “family” relationship, i.e. something like what you see in The Incredibles. But that’s not official, so don’t quote me.