Tch, they're not really planets, right? I mean, if they're not orbiting a star, then they can't have "cleared the neighborhood of their orbit". Yet one more reason the IAU's current definition is so idiotic. (Besides the fact that it suggests that Mercury is more like Jupiter than it is like Ceres.)
Yes, they are planets most likely, because they probably formed around a star and then got kicked out dynamically. This is expected generically in models of how solar systems form and evolve (in particular we think it happened multiple times in our own Solar System).
It is masked but always present. I don't know who built to it. It came before the first kernel.