Helmsman and Spelljammer

I do not know how many of you has ever had this thought, but i have and i
must clear my doubts, so this is my question:
 .In the Spelljammer boxed set Lorebook of the Void is stated that the
spelljammer user feels like he could stand on the deck and see all around
the ship and feels like he/she is the ship, so does he have the full control
of the ship or does he act only as a motive power?
 a)if he is only a motive power, then the one who really controls the ship
is the helmsman?
 b)if he can control the ship himself, then what the heck is the helmsman duty?
 c)if the two can interact and control the ship both, who has the last word
(i.e. who has the power to change the other's decision and guide the ship as
he wants)?
 d)if the spelljammer is in charge, does he have to be near the captain to
hear his orders or can he just sit there in the spelljamming room below deck
listening to the captain shouting in the upper deck as if he was present
(remember he feels like he's all around in the ship..)?

Please answer....