Fridge Logic: Where did Wilbur's Y chromosome come from? Males inherit that from their fathers (females have XX).
If Wilbur is anything like Helen Vaughan, then he might not actually be biologically male—he could have assumed a shape agreeable to most people's image of physical maleness, just as Helen Vaughan wears a pretense of physical femaleness but is, in fact, an eldritch monstrosity that changes sex according to the situation.
Narm: Describing a fight with an invisible monster is hard enough to do seriously in third person narration. It becomes even worse when the reader's viewpoint doesn't actually make it to the climax of the story and is instead forced to settle for listening to a description of the fight from a guy watching it through a telescope, making the ending come across like listening to someone describe a bad mime routine to a blind person.
The Woobie: Lavinia. She's unattractive, deformed, isolated, lives in squalor with a batshit insane father, and is eventually killed by her unholy spawn whom she had begun to fear. To make matters worse, it's all but stated the demonic whippoorwhills managed to capture her soul.