I think A-Rod is leaving because he can. He doesn't like it there, and can at least get the same money somewhere else. People keep saying "Boras must know that they can get better money from another team" but I don't think it has to be better. And that answers the second question.
The best way to sell A-Rod is not on his stats, as impressive as they are. The best way to sell him is on his potential economic impact to a team. The best way to emphasize that is to make sure that A-Rod is the story in baseball. That would not have happened if Boras had waited ten days past the World Series to announce A-Rod is opting out of his contract. Nope. But by announcing it while a lot of people were still paying attention to baseball, i.e. during the World Series, then he smoothly transitions all those World Series watchers into A-Rod watchers.
Boras doesn't have a deal in place already. He's going to have to work for his money, and this is part of how he's going to do it. His client wanted out of New York, and he's just doing his best given that situation.
So all the idiot Yankees fans (which is not all of them...) are getting their wish. No more A-Rod. The Yanks have a lot of money to spend on replacing A-Rod, so that should make for an interesting offseason in the Bronx.
For what it's worth, A-Rod has clearly made the right decision. Any other city would have worshiped A-Rod unconditionally before this season, and would be ready to rename their city after him at this point. I should know, I live in the Bay Area. Barry Bonds is the worst player to root for in baseball history since Ty Cobb, and he is completely adored in San Francisco. A-Rod is nowhere near as unlikeable as Bonds and is almost as good (especially if you consider position.) Plus look at the "new" ownership of the Yankees. They parted ways with Joe Torre and now A-Rod, and in both cases the owners felt the need to talk trash to the press. How old are these guys? It's just amazingly juvenile.