I'd keep Dempster as the #6 guy. Right now, none of the young arms have showed much of an ability to start in 2014. 2015 is a more likely option for some of the better non RDLR and Webster arms.
I wouldn't give $100M (8 years @ Napoli $) to a guy who has never swung a bat in MLB.
It'd be better to overpay and shorten the years to 3 just in case he flops.
Getting homefield advantage is the best way to make sure you play more games at home than away. (excludes WS which is determined by a dumb and outdated scrimmage that shouldn't count for anything but does)
They do it to increase the likelihood of a DP and cause a force out at home. With 0-1 outs, they'll almost always walk the man. With 2, they would be less likely to do so.