that one works the best for me, also.. . He'd been wanting to kill Lynn for a while, lots of pent up rage, lots of sarcastic singing at her, and so on,, lots of pushing and shoving... …It doesn't satisfy Occum's Razor. But it isn't impossible.
More plausible for me though is that Chris has hit Lyn in a rage and 'accidentally' killed her. He's then rung Paul in a panic, and Paul and Chris have removed all sign of it occurring, including Lyn's body.
I just don't see Paul hesitating about helping him, nor about keeping up the story for life, in regard to Lynn and the "cult thing' , either. or Marilyn,, as well. It suited their purpose.