Brace, no need to get nasty. The legal position is that accountability (responsibility) cannot be shared, fact.
Not trying to be nasty at all, just my views on the subject, which I think are fairly similar to yours.
All I’m saying is people are trained to behave according to certain rules. In my company briefings are adapted in training to make this clear. An FO is not allowed to initiate the reject at high speed, a captain in LIFUS and a training captain are. It is briefed up front and clear. During upgrade LIFUS, the upgrade simulator training program is not finished.
It all comes down to training programs and company specific procedures.