How could Julius be fooled like this in The Secret Adversary?

Question

Warning: spoilers follow for the entire plot of The Secret Adversary by Agatha Christie (freely/legally available to read online).

In Chapter XIX, Tommy and Julius arrive in Manchester following a summons from Sir James Peel Edgerton, who claims to have found Jane Finn. They talk with the young woman who's just recovered from an accident, and she gives them information on where to find the crucial oilskin package and documents. A description of this scene caught my attention at once:

Somehow the whole scene seemed unreal. It was so exactly what one expected that it gave the effect of being beautifully staged.

Indeed, it later turns out that this "Jane Finn" was a fake, planted by Mr Brown and his people. (For what purpose, I'm not sure? Just to make Tommy, Julius, and even Mr Carter call off the search for the documents, assuming they'd been found by Mr Brown after the events of Chapters XX-XXI?) Anyway, what confuses me is how Julius could have been fooled. This was one of the points that made me think he might be the mole, and I was well and truly sold on that theory as soon as it emerged in the book, right up to the moment that the true Mr Brown revealed himself. Given that Julius was in fact exactly what he seemed to be, rather than an enemy mastermind in the camp, how does this scene make sense?

Julius had seen a photo of Jane Finn, and by the time of the meeting in Manchester, he'd even recovered that photo secretly from Mrs Vandemeyer's safe. If he knew what the real Jane Finn looked like, how could he be taken in by a fake? It couldn't be that Mr Brown had found an exact doppelganger, because then Tommy would have noticed that she looked like the girl Annette whom he'd met by that time.

Answer 1

Julius has never met Jane. All he has to go by is a single photograph from years ago. It's not particularly surprising if someone's appearance in real life differs, even quite markedly, from a black and white photograph taken a long time earlier.

And presumably the girl playing the part of Jane, even though not a doppelganger, looks enough like her to pass muster under the circumstances. In Chapter 20, Julius says:

She’s not like her photo one bit. At least I suppose she is in a way—must be—because I recognized her right off. If I’d seen her in a crowd I’d have said ‘There’s a girl whose face I know’ right away without any hesitation.

Part of Julius's finding the girl's face familiar is that he has indeed seen her before. In Chapter 21, he realizes where:

“A nurse’s kit! Gee whiz! I must be going to Colney Hatch! I could swear I’ve seen Jane in a nurse’s cap too. And that’s plumb impossible! No, by gum, I’ve got it! It was her I saw talking to Whittington at that nursing home in Bournemouth. She wasn’t a patient there! She was a nurse!”

This notwithstanding, I imagine we're supposed to assume that there is enough of a resemblance between the spurious Jane Finn and the real one to make the substitution plausible, particularly given that Julius does not really know what Jane looks like.