The Thing in the Basement Takes One Step is a very particularly kind of creepy (spoilers)
This whole story feels like it was written by someone intimately familiar with paranoid schizophrenia and/or psychosis, and puts that uncertainty onto the reader. Coincidences are on purpose, everything happens at a suspiciously perfect time, someone else can get inside your head and read your mind or control your body, but I think Paul has the best parallel here with a small detail. A common coping mechanism people use to distinguish reality from hallucination is to use mirrors. Schizophrenia essentially hijacks the brain's pattern recognition and auto-fill making it's hallucinations incredibly realistic, conjuring fictitious people you can have entire conversations with. But, reflections are abstract enough that the brain has a hard time filling them in properly to fit the delusion. Don't know if someone is real? Look in the mirror, and if the reflection is wrong (or not there at all) then they aren't real.
Paul has a mirror facing his basement door.