What cover up? The incompetence of the PD took care of that. They overlooked a lot of evidence, declared it a suicide and washed their hands of it.
This is easily done when the target is an actual wastoid who has not been planning for that exact scenario for over 30 years.
I'll still think it was suicide. The dude had options–right up until the end. The fact that the woman that he thought was the love of his life didn't tell him what those options were, sabotaged him from behind the scenes, and if she didn't pull the trigger, she probably bought the shells... well, it's like this:
Suicide takes many forms.