I know this is going to be very unpopular but I really think Jack committed suicide.
The fire is too obvious, we know it isn't the fire.
Kevin makes reference to throwing his model airplanes out after his dad died, but the house looked to be a total loss, would those have even survived a fire?
We see Rebecca driving back to the burned house with Jack's things (we assume Jack's things) and she looks angry, maybe the fire was the straw that broke the camels back.
We saw Kate get angry with Toby for making some suicide references.
Kate blames herself for his death, maybe she thinks its her fault she didn't see it coming?
His death was obviously traumatic, something they still don't want to talk about and hesitant to discuss his flaws at all.
Then there is Jack himself who I think battles with a lot of demons with a huge smile on his face.
The loss of the baby, his abusive father, losing his brother, possibly his house, recovery, plus the stress of wanting to be the perfect father and husband. My uncle reminds me alot of Jack. Always a big smile on his face, loving husband, wonderful father, very involved in his church. One day shot himself in the head and we still don't know why, no note, no signs. Sometimes it is the ones you least expect.
What do you think?