That's actually three questions, but I'm happy to answer them all.
First of all, you ask how Bella can be in love with two people. The answer is rather complicated and I understand exactly how you feel about the unfaithful relationship.
Bella doesn't realize that she loves Jake untill it's two far for her to pull away without pain. It's her lack of self understanding that puts her in such a sticky situation. If she had realized earlier that her feelings about jake were deeper than just friendship, she would have pulled away.
I recommend this page if my explination is not clear enough
http://www.stepheniemeyer.com/ecl_faq.html
Jake runs off to... nowhere. He just runs. It's not some crucial plot point or dramatic twist to the story, so I wouldn't linger on it.
I havn't pre-ordered breaking dawn, but I plan to in the next couple of months. I can't take the risk that my town's pitiful bookstore might not have it on the day it arrives.