1) Yes, you can PhotoRead them back-to-back. Make sure you are clear on your purpose for each before the PhotoRead in order to get the best results. When I do books back-to-back I'm usually doing books that are related.
2) I would prepare and preview, but do not spend too much time on it.
3) I tend to PhotoRead most books in two sessions. If I PhotoRead to page 100, when I do the next session I will start at page 90 to make sure I'm up to speed when I hit 100.
4) I PhotoRead most books multiple times. Typcially I will PhotoRead a book on day one and activate it on day two, but I will PhotoRead it again before activation.
Every time you PhotoRead a book, you are getting it as it is relative to you at that point in time, which adds more depth to your understanding of the material.