Firstly, you can pretty much flash any firmware for your phone model on to your phone. You don't have to go through each version of Android.
1. Not sure
2. Depends on which CSC you flash. Certain regions don't have access to certain apps on the market place. Flashing a different CSC to the region you are in could effect the quality of phone calls
3. No idea
4. They are not the same version. Samsung release software and then it's distributed to network carriers etc. Network carriers can then put their own software on top of what Samsung has given them.
5. stop using sum1 and b4 etc.