1. yes http://www.wireless.att.com/blackberrystart
(you can not migrate your account nor rules between carriers - sorry)
2. Yes assuming you don't do insane things like bitorrenting over your tethered connection, they do watch(so I have heard)
3. Not sure
4. a bunch more. Does your company have a nation wide account? Get a the fan # or use your email and see if you can get a personal discount. Otherwise you will be paying a bunch more. Tmobile is sadily cheap for a reason, great cs, horrible coverage.