Sorry but this is absurd in so many ways (SPOILERS)

Have you even paid any attention to the language or even subtitles?

"- Why didn't Vera say a thing when she got shot? She tried to open the door Félix was trying to lock, but she never spoke her voice. "

***SPOILERS***
Felix shot Vera between her neck & mouth, so she couldn't just speak right away, she was choking on her blood, at the end she was barley speaking because she was ignoring the pain & accepting her demise.

"- Martin was supposed to be in Peru or wherever and come back in a week. How could he be in Felix's home if he didn't make it past the tunnel? And using his razor... "
Martin was NOT in Peru, didn't you hear Claudia & Bruno's conversation after the police station? Martin was missing for 2 days & they alerted his disappearance, they only told people that Martin was missing because it was her business only.


"- Nobody noticed Felix at the party as someone who wasn't part of the initial surprise. Was he supposed to know it was Claudia's birthday? Did he even know it was her house? "
The guests weren't all friends with Claudia, they were probably Bruno's friends & he invited a full house in order to make Claudia feel as if she wasn't alone, everyone probably didn't know everybody, so Felix blended in perfectly.


"- When Felix entered Claudia's house and she comes in with Bruno, why didn't Felix show up, explain the situation as this was supposed to be the house of the man he saw in his own house, show them the drawing as a proof, then go back at his house with them only to finally find it was his ex wife he got locked? "

Because Felix thought that he already had Martin trapped in his attic, so he went to Martin's house until "Martin" died from the wound Felix left him, he later became attached to Claudia & decided to help her & hopefully pursue a relationship with her