Welcome to the Scrivener club, and the Mac club at that. I think that, to some degree, how the actual content of the Draft unfolds is going to be different for every author. For myself, when I have done NanoWriMo in the past, I didn’t worry too much about structure in the Draft itself. I wrote all over the place and let things evolve as they were written.
With other applications, it would be more difficult to keep track of how many words had been written, using a shotgun method like this, but with Scrivener you have two tools that can come in handy. The daily session goal, and the overall target itself. Both can be accessed with Cmd-Ctrl-T, and set from that panel as well. If you have downloaded the NanoWriMo version of Scrivener from the promo, then it’s already set to 50,000 project goal for you. You can set the session goal to 1,667 to use the most common daily target and it will keep track of everything you type, wherever you type it.
There is one catch, it is a session goal, not a daily goal. If you quick Scrivener and come back later in the day, the progress will be reset. This is where the over-all goal can come in handy. With a little math you can figure out where you are supposed to be in the entire project for each day, and use that progress metre to make sure you are on target.
So while you could keep daily documents, it isn’t absolutely necessary for record keeping purposes. If you find writing into an evolving outline to be more natural, go ahead and do so. I found with my own Nano projects that initially a daily writing document was easier, but that after the half-way point, I ended up doing much more in the way of “shotgun writing”, putting paragraphs and scenes all over the place, and it would get to be a bother to have to note down where everything is suppose to go and then re-factor it all at the end of the day. Back then I had no choice, I wasn’t using an application like Scrivener that makes daily writing goals much easier. Now that I’ve done a few Nanos in Scrivener, I wouldn’t go back to that method.
Since the deadline is tight, a good thing to do is take advantage of Scrivener’s best features. Write small parcels of text instead of large chapter-sized documents. Since the book outline will be evolving rapidly as you write, it makes this evolution process so much easier. Use Edit Scrivenings to get larger views of your text, and make liberal use of the Split and Merge documents features if necessary.