If JO and KL did not know DG existed, and let's say DG's target was AL, why in the world didn't he go after AL when he was alone somewhere, sometime?
Why risk trying to murder two people instead of one? Again, more risky not knowing who was at the house (which turned out to be three persons sadly), but could have easily been 4-5 people also if CH and her husband happened to stay the night at the last minute.
Also, if KL and JO (and that means NO also) didn't know DG existed, they would not recognize him so they could have been spared, especially NO. Nathan would not have recognized or known DG, he would have been a stranger to him that night.
Kathryn must have known about Douglas. Douglas premeditated the murder of Kathryn and Alvin. Jen can't speak for what her mother knew about her step-children's families.