At first I thought your dad was doing coke with his brother that night. Then I reread your posts and realized you never said that.
Do you remember that night? Did your dad act odd or distracted or anything like that? Or perhaps much more eager than usual to interact with you? If he'd set up something to happen to his brother--whether he intended that something to be death or just a bad drug experience--it stands to reason that he might do one of the above.
BTW, your posts of the police reports need some more redacting. First names, the last name of the witness, and place names appear in a few places. It's not enough to identify these people to complete strangers, but if anyone reading this knew them, and knew about the cocaine overdose death, they would probably recognize them.