Well you can get most of the details of "How was the order of the Qur'an decided?" here.
Why is the Qur'an divided into the 114 surahs?
According to the answer which I'm pointing, it seems it is pretty clear that Prophet Muhammad (pbuh) himself divided Qur'an into Surahs & not scribes. It is simple enough to understand 'why' if you read them, you can simply find that each Surah is a complete chapter all by itself in a normal way like any other book is divided into chapters. It is divided such that it retains the meaning within each Surah along with its context. And also, we can find/observe, within each Surah, there is definite way of starting and ending, implying the Surahs were divided as such by Prophet (pbuh) under the command of Archangel Gabriel.
Here is are small examples 112, 113, 114 the last small Surahs, which are complete by themselves not depending on each other.
Does each surah represent a single revelation to Mohammed by God, that is, a single visit from the angel Gabriel, and the words of the surah are simply a direct transcription of what the angel Gabriel said?
No No, not at all. It is believed that the first Surah - The Opening (Fatiah) is the only longest chain of verses which was revealed all at once, I mean at a stretch, the way you're thinking. Just imagine, there are 286 big big verses in the second Chapter, how come in one go?
Or, is the organization into surah "man-made", something that came later?
No, 'within' Surah, the order was set by the Prophet (pbuh) himself, but within Qur'an, the order of Chapters was 'man made'.
May the creator guide us all.