XLink reflects this fundamental law of the universe in its data model and syntax. RDF, which is not primarily designed for asserting relationships, does not. Both are valid choices in the differing domains. Given infinite computational power, yes. This is, for instance, exactly how the relational model works.
All relational relationships are reversible.
A particular programming language may only be able to traverse in one direction but there is obviously a bidirectional relationship. In an GCed language, object Y cannot be destroyed until object X is. One can imagine a language where it is possible to ask for all of the referents of an object. The question is whether you get access to it. The XLink data model does.