Sorry, reposting because the formatting is messed up and I can't delete it:
I think that Lysander is Andromeda, that's the only way the book makes sense to me. Hear me out...
Lysander is the only individual that connects Vienna (where he was recruited by the Austrians), all events and other possible suspects.
Lysander is described as a German/Austrian wolf early in the book.
He personally instigates the deaths of all the people who could expose him - Glockner, Vandenbrook and his mother.
The only person who would plausibly use the Andromeda libretto as the code is Lysander.
He lied to Madame Duchesne about the libretto code because he is Andromeda. She was right to identify him as a traitor and shoot him for it.
Vandenbrook realizes Lysander is Andromeda just before he is killed. Vandenbrook's actions are always consistent with this.
Lysander lured Vandenbrook to his death, but claims it was suicide and hides the meeting from his spymasters.
Lysander set up the theatre meeting between Vandenbrook and his spymasters as a plant for his story.
In the penultimate chapter there is a jarring exchange between Lysander and Munro about the letters being posted from London. Lysander realises he slipped up and wants to know the source of the information. This part sticks out from the rest of the chapter, revealing the twist about Lysander being Andromeda just before the end of the book.
A core part of the book's first section shows Lysander's character with him lying about the gardener's son.
The quote at the start of the book explaining its premise: "Truly, to tell lies is not honourable; but when the truth entails tremendous ruin, to speak dishonourably is pardonable".
The last sentence of the book is "it's hard to make things out clearly, hard to tell exactly what is what, and who is whom - Mr Lysander Rief looks like someone who is far more at ease occupying the cold security of the dark; a man happier with the dubious comfort of the shadows."
The whole book is Parallelism, where Lysander twists his lies to make a new 'truth'.
Does this sound right? Or have I not had enough sleep?!