Personally, I do not think he could (especially from a sitting position) possess the strength to crush the side of her skull with a blow from a hand. "Blunt force" doesn't usually mean "human hand" anyway.
The blunt weapon damage on her left side certainly points to a possible in-the-car attack, but it could also just mean an attack as they’re facing each other, (if he’s right-handed.) If it was in the car, it’s a little surprising that the DA didn’t mention blood or any sign of violence. (Or did I miss that? However, there are blunt weapons that might not break the skin.) I think there’s still a reasonable possibility that she went to his house willingly. Their communications will probably clarify this.
MOO