Was Hess yet Hitler's heir for the Nazi Party?
Hess was never Hitler's designated successor. He was Hitler's deputy for Party business, but that was not the same as being "Vice Fuhrer".
At the time of the Enabling Act, Hitler's legal successor would have been the Vice Chancellor, von Papen - who wasn't a Nazi.
When Hitler did designate a successor (in 1939), it was Goering.
In any case, in this scenario, Hitler is not dead, only disabled; the question is who becomes "regent".