I will add to you couple of things to your observations. Long time reader, first time poster so bear with me.
1. If my husband, children and I, had recently moved into a new house and received a threatening letter by a self proclaimed "Watcher", I would at the very least contact the police as a heads up kind of thing. In this day in age, I wouldn't have simply shrugged it off.
2. I would immediately call the seller to find out if they had any experiences with the "Watcher." Possibly the sellers could lie, nonetheless I'd ask.
3. As aforementioned, I would have installed security cameras outside and inside and other high tech security equipment.
4. How did the buyers know the sellers received a letter 'on or about the week of May 26'? I think the buyers said the "Watcher" mentioned it in the letter to them. The specificity of the date is strange to me. Why would the "Watcher" not just say, "I've sent a message to the Woods too." As opposed to giving the approximate receipt date of letter? The main point of the Watcher's letter to the buyers would be to let buyers know the seller recently received a letter too. The exact timeframe of the letter doesn't seem necessary. An approximation of the timeframe-yes but exact timeframe-no.
5. Why would the "Watcher" send a letter to the sellers first? Wouldn't it be more important that the current buyers know of the Watcher's self proclaimed "rightful ownership and possession" of the house? The sellers are no longer pertinent.
6. Now we have the third generation of the "Watcher" watching the house. Decades of watching, wouldn't there be some whisperings and/or rumors going around the neighborhood and/or the town about this supposed "Watcher?" Especially with the proximity of the List murders.
7. Were the buyer's cell records examined to see if their phones pinged on a tower in Kearney where the letters were postmarked? I know cell record evidence is controversial, but it would at least rule out that the buyers had been near the mailbox location.
8. Did either buyer mention receiving these letters to family members, friends or co-workers? If I'd received a threatening letter I'd immediately tell my sister, friends etc. Simply for the sake of discussion, but also in case something happened to me. I would also ask the neighbors if they knew anything about the "Watcher."
9. Did the buyers warn the construction workers about the possibility of evil doings by the "Watcher" so they could be on alert?
9. Have they done a forensic study of buyer's finances? This goes to the "in over their head" theory.
10. Was there DNA evidence found? Fingerprints on stamp, envelope, and/or letter?
11. Buyers only received 3 letters in year? If I were the "Watcher" and my family had been consumed/obsessed with this house for nearly 100 years, I'd be writing a whole lot of letters.
12. Were the letters handwritten or typed? Goes to have police doing forensic examination of buyer's computers, mobile phones etc.
13. It's hard for me to imagine that one family could have such a vendetta, hatred, obsession against a house and its' occupants that it would consume them for nearly 100 years.
14. I just googled to see if I could find any fiction book (as I type that it could be a non-fiction book too) had a similar plotline. But didn't find anything.
I don't buy the buyer's story. The evidence or lack thereof doesn't add up to me. Something is rotten in the state of Denmark.