A: all content will become re-available when you create another character, but not when you switch class. just think of your character as someone in real life: you could change your job from being an accountant to a firefighter, but if you have completed the quest of helping Mr. Smith for fetching food for his wife, you cant complete the same quest again; on the other hand, if you create a new character, then you are a brand new person, so all content will be reset again.

A: yes, as explained above.

A: yes, as above, meaning you could be the jack of all trade and master of everything.

A: yes, as above.

A: it's up to you. some ppl like to get to high level asap; some ppl like to experiment with everything. note, however, when one of your classes is higher in level, your other classes which are of a lower level receive exp boosts, which i believe is in tandem with the difference in your highest class level and the level of the class you are currently lvl-ing.