The story of the Siamese cat attacking an intruder sounds 100% believable.
Years ago my brother in law and sister in law had a Siamese cat named Tara, who in my opinion was "meaner than a junkyard dog". But she was very loyal to bil and sil.
Well, one day when a visitor was there, and the owner and visitor were just standing in the garage, the visitor pointing to a piece of furniture up on a shelf, suddenly without warning, Tara jumped on visitor and seriously injured her, sending her to the hospital for stitches. There was a lot of blood at the scene!
Since then, it's been hard to trust Siamese, but they sort of have the reputation anyway.