I love when you write much Anne, you have an amazing mind
(honestly)
Okay I´ve got another theory (and I have to admit: a not likely one)
What if John was the one who killed the mother. I mean we now nothing about him and he has a very scary double oh and after the house burned down in the pilot did you see the look John had on his face...scary.
So let´s say Mary was pregnant with Sam and was about to lose the kid, John knew this and made a deal with the devil, Sam doesn´t die and in exchange he sacrifice his soul. So after John was a few weeks old the devil possed him and killed Mary (why? I still have to figure that out) maybe he wanted John to suffer and maybe something happend after Mary´s death.
And maybe because of the deal and John unwillingness to sacrifice himself the devil goes to the next best victim. Sam
So Sam hot possed in the pilot and he is the one who killed Jess.
Okay I just read that and It sounds super stupid
But I´m having a bad cold so please be leniently