Step Turn: in answer to your query about airline check rides. If the checker is occupying an OPERATING SEAT ( versus a jump seat) then he will be P1 regardless of relative seniority.
If you think about the possible scenario of a check ride which is failed by the candidate then you still need a qualified crew ( ie. P1 + P2 ) to operate home or subsequent sectors. In other words a "failed" P1 pilot can be legal to operate second pilot.