2.) If you are planning to unlock the cores only at first, then the stock cooler is fine AFAIK - correct me if I'm wrong, but it's pretty much the same as the Phenom II X4 heatsinks.
3.) AM2, AM2+ and AM3 use the same bracket.
4.) The Phenom II X2 555 performs around the same as a Core 2 Duo E8300 or Pentium G6950.
5.) Set ACC (Automatic Clock Calibration) to auto, or something similar.
6.) If your CPU is unstable, it either won't boot and revert to normal settings, or you can access Windows, Prime 95 and if it fails one or two cores are unstable, you would be better off asking someone who has unlocked it, hopefully someone does.
7.) In terms of what? You can get an Athlon II X4, which would be better for multitasking, but worse for gaming. Then you got the OEM Phenom II X3 720 for $104.99, then you'd have to buy an aftermarket HSF.