No, I assume it was murder because the damage to the skull was extensive

Not really. He had an abrasion on the back of his head and had skull fractures consistent with falling and hitting the back of your head. Was he a tall guy? Striking someone in the head leaves different injuries and tell tale signs than blunt force from a fall, and no such signs were found. Now could he have been shoved to the ground, backwards down some stairs by an intruder, resulting in his injuries. Sure. But again, less likely.

I believe he was hit on the back of the head with a sap, or a similar weapon, for reasons unknown.

Again, being struck by an object in the head typically leaves characteristic injuries that weren't found.