While every helicopter I've flown requires the pilot to be in the right seat for solo flight, none is impossible to fly from the right seat if the controls are installed, and there is no rating for left seat only. For aircraft requiring two pilots, both have to be qualified to fly from either seat. There is no requirement for a pilot to die on a checkride, although it's not unheard of. The PF is simply told to act as if incapacitated, and the other pilot takes control. Either pilot, in either seat, could be required to take control, and it's assumed that he can fly the aircraft, he just has to recognize the incapacity and take control. It's a CRM issue.
AFAIK that's not a regulatory requirement, because it doesn't happen on every checkride.