@Eaglebaby,
You wrote: "Hearing a door open and close is no proof for someone having left the room."
My understanding is you can hear on the CVR the capt imploring the f/o to let him back in (along with knocking, pounding, etc.). Also, with area mics being what they are, I'm sure not only that they (and the CVR analysts) can determine the identity of the voice on the other side of the door, but also whether one or two human beings are breathing in the cockpit.
I really don't think the theory that the capt and f/o were together throughout / that the capt is responsible instead of the f/o holds much weight at this point.