|Hello, Jayne -- I enjoyed reading your story. It's got what I'd call good bones: the structure is sound and well serves the increasing tension of the plot as the scares mount for Jessie. The characters (including Shannon!) are believable.
I'd have liked to learn more about Jessie, and I think you could get away with adding a few more details about what it's like having just moved into a new house (perhaps there's a fresh-paint smell, or there are only a few glasses and dishes unpacked, so she has to do a lot of washing-up as she goes, or something like that).
What I wasn't clear on was Justin's motivation. If he simply wanted a companion for his dead dog (nice creepy touch!), he could have just stolen and not returned Shannon, rather than returning her and later coming back to Jayne's house. You do nicely suggest early on that he's interested in Jayne with his overly direct inquiry regarding whether she lives alone, and of course his being more interested in Jessie than Shannon is congruent with his going so far as to invade her house, but then when he does intrude, his talk is mostly about the dog. (I liked how Jessie improvised, trying to defuse the situation!) But still later the police allude to his stalking the former owner of Jessie's house, so it seemed like he was in fact desperate for a woman. Anyway, I guess I didn't quite get how this psychopath could be after both the woman and the dog.
Finally, regarding technical matters: I did notice some punctuation flaws -- missing apostrophes and commas, principally -- which you'll want to fix at some point. As for the adverbs you mentioned in your introduction to the story, I'd say you were admirably sparing with them (though I thought "resolutely" was not quite the right word there, and probably unnecessary in any case). One thing to watch out for is having characters address each other by name -- people rarely do that, actually.
These are just my reactions for whatever they're worth. Thank you for sharing a nicely nightmarish story. Best wishes in all your writing endeavors. -- S.