Holy crap great questions. Welcome! Be sure to check out The Ultimate Beginner meta-thread for a good collection of resources, the welcome thread for forum tips and how-tos.
1.) That’s definitely a personal call. Like Wheylous says, it all depends on how well you understand the basics concepts.
2.) Wish I had time to but haven’t started it yet. From what I can see just from the free stuff, I’d say it’s gotta be worth it. It’s Tom’s baby and he’s definitely passionate about it…plus he’s managed to bring in some heavy hitters to make up the faculty.
3.) That’s definitely a personal call as well, but my recommendation would be as an introduction or epilogue to each chapter. Read the study guide for that chapter, and then the chapter in the book…or vice versa. If you think you can handle the chapter pretty good on its own, you might save the study guide part for after…as a recap for what you just read. It can help you digest and retain the info.
Other people might find it more helpful to read the study guide chapter first, as it will help give you an idea of what you’re about to be reading in detail, and give you sort of a heads-up for what you should be expecting to learn and things to keep in mind as you’re reading the corresponding chapter in the book. You might try it both ways…read chapter one in the book, and then the study guide for that chapter. If you feel like it would have been more helpful to go through the guide before having read the chapter, try that for chapter 2.
4.) Wheylous got that one too. We had a few comments about Zinn here.