The dna sample could be unfortunate contamination, thus ruling out the actual culprit.
I would have moved in, with my kids and played the game. I would have started hanging out garden banners talking to the watcher. Simple and friendly stuff. Good Morning Watcher, Happy Easter Watcher, etc. The more letters the culprit wrote the more likely and faster she would be caught.
Also, I think the writer was more likely to have been into Gothic romance novels rather than horror films so I would investigate who borrows that genre heavily from the library or buys from the local bookstore, new and used.
“The dna sample could be unfortunate contamination, thus ruling out the actual culprit.”
Could be, but if there is ever a match to someone with a connection to the house, the neighborhood or the Broadduses, the person needs to be investigated.