Oh, me too-- unreliable narrator is one of my favorite tropes! You've used it before too, so I know you understand the appeal. :D

The cat at the door was deliberately ambiguously worded, so there would be the possibility of it being either. But is is actually a literal cat, an given how Marty is, it's entirely possible that he just saw it as a random (possibly very small and hairy) person. :O

Thanks for reading!