I figured the clothes and letter were a plant. But even so - seems like that would have been a crime of passion, and the letter suggests malice aforethought. It was definitely a big plot hole.
Okay, so it's not just me missing something. Eoin's mother said the handwriting was his, so if a plant it was never explained. It's simply too long a story to read again (so the whole train scene was just to spark guilt in Katie's mind? That's 2/3 of the story!) so the mystery will remain for me.