*Yes, I know some princesses don't fit these requirements, but they are often said not to deserve to be in the line-up, and for reasons included in the poll. You can take that into account or ignore it when you chose.*
I think a character from a book could fit the line-up very well, and it doesn't affect whether the princess can be marketed as well as much as, say, whether the princess sings or has a love interest. I also think a princess should be a princess by birth or marriage OR a protagonist in her film.

I just do not see how this is a factor. A lot of the books that Disney adapted are popular. Also, since being a princess clearly does not matter (Mulan), I do not see why it has to be a legend or fairy-tales. People bed to remember that now Merida is included, a lot of the rules do not matter (including this one). The movies based on fairytales are a minority anyways.

I don't think a movie necessarily needs to be based on a legend or fairytale for the girl to be considered a princess.

Pocahontas is kind of a legend but she's more of a real person than a legend right?

Well, many of these movies, even those who don't follow the fairytale, had to get their ideas from somewhere!

Those are the two most important factors by far, but they skirted the requirement a little bit with Pocahontas and Brave. (that doesn't mean that I think they should include book based movies)

As long as Disney didn't write it their selves I would be happy.