As to declaring emergencies, yes that's perhaps getting away from a realistic scenario for this. However, A is perfectly entitled to declare an emergency, at any time he feels it appropriate. In that case A could ignore any rule he wished
ANO PART 22 AIRCRAFT IN FLIGHT Rules of the Air
160 (3) It is lawful for the Rules of the Air to be departed from to the extent necessary:
(a) for avoiding immediate danger;
However, A doesn't need to do that to let B take control, if A deems that to be the safest course of action.