Okay so it turns out the father started writing anonymous letters too.
Seems to me this makes him a primary suspect for being the writer of all the letters to begin with.
But the previous owner got letters 23 years ago and didn’t disclose this - the current owner didn’t find out until he started getting letters. The owner wrote some letters at the end because he was driven crazy and had to defend himself.
Okay so it turns out the father started writing anonymous letters too.
Seems to me this makes him a primary suspect for being the writer of all the letters to begin with.