b) seems pretty indefensible to me. Physicalism is about scientific reductionism, not philosophical reductionism. The only difference is that "interactions" are included in a) because physics is all about interactions. We don't talk about the force between and electron and itself. Force is something that couples two different objects.I was thinking of definition (b). I see you use definition (a). I wonder then how this approach handles problems like black holes. Black holes cannot theoretically be studied other than an entity with few properties. Unless, of course, we find something faster than light, that can carry information. At least I think there is no other option.
I don't know much about relativity or cosmology; my modern studies consisted of quantum and nonlinear. It might be a good question for the cosmology forum above that we could later link to for our discussion, but I would guess much of relativity reduces to spacetime.