My own point of view is that closure isn't always a good thing to push through for. I never got any closure from my ex-wife for example or from my own mother who I was estranged from at the time of her passing (long story). And at this point in my life, I'm fine with that. There are perhaps thing that they could explain that would shed light on the events of the past but then again perhaps not.
What you may want to do instead - and this sounds a bit morbid - is do up something like what was done for my mother after she died but would have been more profound if done before. A retrospective. Vignettes, photos, stories - written or recorded that show the sort of person who CMM is. Where he finds his Joy. What makes him thoughtful. How silly he looks laying on his back and scrubbing the inside top of the stove perhaps. Moments that were important to him like perhaps the births of his daughters or the first time he cleaned the bathroom after they were playing in a mud-puddle
Then, that is something his daughters could perhaps access without any pressure and it could be something for those who care deeply for him to have as a remembrance. He may well be in favour of that if presented that way.
Just my 2 cents.
On BD H52, W50 T27, M26 S21, D23 BD-9-Mar-16 D-15-Jan-18 Final-19-Apr-18 I am a storyteller. The story may do you no good. But a story is never for the listener. It is always for the one who tells