No idea about legal issues, apart from the fact that safety is a shared responsibility. In line training I train ab initio’s with emphasis on ”SIC” (second in command) so they realise they have to act when they suspect the PIC is ”incapacitated” and jeopordizes safety.
When it comes to a reject, the difference is procedural training: when an FO joins the fleet he is trained to be able to reject a takeoff from about 100kts in a pilot incapacitation scenario. A new captain is left hand seat trained to execute high speed rejects. The training captains are trained to initiate and execute high speed rejects in both left hand and right hand seat training.
As far as responsibility goes: the fact that you are responsible for the actions of others is nothing new. If you don’t agree, you shouldn’t be in training position I guess...