Wow...I was surprised to see this topic still being discussed...took me a while to read all the posts since the last time I added my two cents on this thread.
Anyway, as I've stated prior...I've watched the doc several times. Danny knows more than he admits - whether it be that he knows Diane had an alcohol problem or at the least was still drunk from the night before and would smoke weed and take "hair of the dog" to feel better....or.....that he knows the content of the telephone conversation Diane had with her brother and that it holds the key to unlock part of "why" this happened...we'll never know.
I will say this - the dynamic between Danny and Diane was unique; like her MIL states in the doc..."...Danny was like her oldest child...", that being said, I can only imagine what it would be like being in that marriage, and, with Diane being the "type A" go-go, take charge, get it done, make it perfect kind of person she was...I think she snapped.