My circumstances were very different from yours but I will say that my deceased AH, after years of mental illness and multiple accompanying addictions, completed suicide. His family did and still does blame me for his death. I am completely NC with them, they've also cut off contact with my children. I had gotten preliminary information from the ME, who tried very hard to be sensitive as well as cut off the most disturbing of details, such as hotel room numbers, which at the time I didn't understand, but now I understand would not have served to make any additional sense of the situation for me.
Your situation is different in that it was a medical condition, but similar in that his perpetuation of addiction was a factor; regardless, what has happened is in the past, and the choices he made were his choices and not yours, none of it can be changed, and knowing more details will still not change anything. I'd say for your peace of mind and healing, to remain NC and not write that letter, or if you do, write it for therapy and then shred it but not send it.
I'd also say that the amount of hatred and unkindness that was directed at me from the estranged family was so difficult for me to understand or reconcile; contact with his family might be extremely wounding and triggering for you, and any information you might gain from this, I'm not certain it would be worth it, for your healing or for theirs.