Megan, most of us if not all would expect in the captain is on-board (his intended PIC flight) he would have the ultimate say/control.
That said the only reference we can find of when a captain assumes "legal" command is from regulators and the Chicago Convention listed above (doors closed till opened).
Some or all company's may contain this in their SOPs, but none have been produced to see or known 100% by staff concerned.
1 poster said he as pilot was listed as responsible "but" it was confidential company document so could not post it.
As this is safety sensitive it should be commonly known, but is not and still to date no proof of even an old leaked SOP from an ex employee been posted.
That all leads to me to think it has been missed in SOP's and by regulators. It happens.