1) The main quest its different in each class city til level 15, where it merges and it becames the same for every class
2) nothing really, but a job its a advanced class, with some new skills besides the base class ones
3) some are, some arent, you can check out which.
4) yes, but with the same character
5) you just need to do the quest, a job its but a stone in your armory. You can switch it on/off whenever you like!