We have always played that non-magical arrows fired from a magical bow are treated for all practical purposes as an extension of the magic from the bow.
May not be RAW, but that's how we play. Otherwise the value of magical ranged weapons is much reduced.
Even though it's not explicitly stated, I think the reasonable answer is yes, the bow imparts its magic to all ammunition fired from it. That's the point of enchanting the bow. If you enchanted the bow with ghost touch but not the arrows would you not expect the arrows to function as if they had ghost touch?
No, you shouldn't need +1 arrows to hit incorpreal creatures. Enchanting ammunition has the bonus of being cheap and being variable, allowing you to have some arrows with fire or acid or electricty or cold damage without having the other properties on them as well.
I would assume it's very likely that the intent of that quoted rule was to cover all situation in which a magic weapon is require to harm a creature. Whoever wrote that would likely not have remembered that incorporeal creatures do not have infinite damage reduction, but that it's technically a different term that applies there.
But I see no reason why anyone would want an arrow to be treated as magic in one situation, but not in another. I am all with Adamantine Dragon and would allow it.
