You are right about Q. The great Pauline scholar Ernst Käsemann, I am told, used to tell his students, "Every morning when you wake up, repeat 'Q is only a hypothetical document.'"

Nevertheless it does designate materials that Matthew and Luke have in common that Mark does not.

As for early provenance of John, it is a thesis more and more accepted because of the author's intimate knowledge of Jerusalem, Pilate's HQ, the Temple, etc. And this rests upon Papias especially, who knew personally some disciples and eyewitnesses, but also internal archaeological evidence. Richard Bauckham is particularly convincing on this point.