I dont think we actually find out how he died

Perhaps Andrew became a priest because Frank became a lawyer, younger brothers often want to take a different path from the older e.g. Mike

Sandra Corleone should have been given more lines, and an appearence in part III, perhaps a meeting with her husbands bastard son.

Yes great point about Tom not being killed because the Hagen family are still about HOWEVER,the promise of Pentangelis family being looked after was what drove Frankie to slash his own wrists, sooooooo maybe???