I think you are complicating something that is very simple. There is no money left. He spent it all (almost all) on lawyers. He wants her dead and her body cremated. That is the only way he walks away clear. If she could talk or if an autopsy is performed he stands a chance of being caught.
This is a red herring. She will never ever be able to talk short of a miracle. Whatever so called evidence there was it is long gone. I do not believe he tried to kill her anyway.