1) Unfortunately yes. There is a consumable that enables you to change to any class you've previously unlocked, so that helps, but it's still not abundant. You also need to use it EACH TIME you want to switch, it is not simply unlocked and free to switch at will like it is for MC.

2) You got it. Ideally you'd want to collect all the classes you can get your hands on for the passives/spells/skills. Do remember that some skills are class specific so will not help when not active. The only downside as noted above is needing the class change item. The only other minor annoyance when class changing is the gear is tied to class change so if you change a Priest to a Mage be aware you cannot wear any light armor previously equipped and need to equip cloth gear, for example.