I think there can't be an answer.
The contradicting newspaper and postcard give two ways to understand the story:
(a) as a realistic story from the viewpoint of a (poor) insane person. Not unlike other stories. Here, Scout indeed exists just in Eric's head, as A Arnold said, and can "know" anything Eric knows.
But then there's the postcard.
The stamp and postmark show it was sent from Greece, where Eric (or his dying mind) ultimatly retires to.

Did Real-life Eric fly to Greece, send the postcard and got back to Manchester to die in a dump? It's not just unlikely but impossible. The newspaper clip mentions a "large-scale police manhunt" after Eric. Even if that's inaccurate, I still can't see an insane missing person passing passport control from the UK and back. In the "realistic-psychological-novel" version, Eric probably spends his last weeks hallucinating in Manchester's "unspaces", with his imaginary friends and foes.
So who DID send the postcard?
(b) This must validate the Fantastical reading, in which the mythology is real and unbelievable things do happen as Eric describes them. This reading can easily settle, say, the postcard: Eric's body could be found dead while Eric's soul/mind/whatever lives on in the other dimention the story ends at. And since all the metaphysics in this universe are about scribe and written language, it's plausible that Eric found a way to communicate, through written form, from his "beyond". It's plausible that he sent the postcard from another world.
But Scout isn't plausible. If, in that fantasy/occult universe, Scout is as real as Eric, than how did she know about, and resemble, Clio in so many little moments? Let's even say that in the story's climax there's some kind of weird rapture, and all of a sudden Scout has always and ever been Clio, or something. After all, in a fantasy story one can think up fantastic stuff, right?
Then how do you explain the tattoo? The very intimate bits of knowledge Scout seems to "unknowingly" throw in all along the story? Did she gather facts about the First Eric in order to use them like a con-artist would? Or maybe it's a ghost/dibbuk story, and it's dead Clio's soul connecting to Scout's body?
These answers are of course silly. Eric could also be fooling Randle all along, or be himself a ghost. There are endless ugly "deus-ex-literary-machina" solutions. But it's just that kind of story, is it.
As far and deep as I can see, Scout's relations to Clio don't make sense. too many things don't fall into place for neither the sci-fi or realistic solutions to be elegant. Taking all the facts in, both options don't make sense. And that's beautiful.
Questions
page. But don't worry too much about it, people should know to expect spoilers here anyway.