Well, bear in mind that I have a bias--I think he probably did it. That said, IF he did it, he had to sneak into the house to do it, right? Who knows how many times he did sneak into the house, and skulked around listening and watching. So if his confession is true, it would be possible for him to know some things about the family.
he would have had to sneak into the house, and planned it to be a kidnapping. he writes the note first with material he finds in the house, it takes him a long time, even writing a practice note. writing a ransom note after he had killed her, makes no sense, so the note had to be written first. where did he get the information about the $118 sum? then, what happens? he abducts her from her bedroom, but doesn't leave the house right away, instead taking her into the storage room in the basement first where the murder occurs. then he leaves the house.
there are many loose ends here.