question about the letters (spoilers)
If Gomex wrote the letters and sent them to the victim, why were they in his mother's house? Did he steal them back when he killed her? If so, why did he hide them in his mother's house? (I am assuming that was his room, though it looked like it was the mother's room.) If he wanted to get rid of the evidence why not burn them? Emotional attachment?
If that's the case, why would the victim have kept them? Did her husband know about them? In which case, shouldn't he have already been familiar with Gomez's name when Esposito came to talk to him? Did the victim keep this a secret from her husband?
Could it be that he wrote them but never sent them?
If he didn't steal them, could it be that they were marked'Return to Sender' or 'Addressee unknown' and returned?