Couple of questions

[Sagittarian]

You can pick up as many classes/jobs as you want, so don't worry about that part. You just need to register at the guild in the appropriate city (see Enkidoh's response).
To unlock Scholar as an Arcanist you will have to get Conjurer to 15 anyway. If healing is what you want to do, I'd say start Conjurer once you unlock the airships and get to Gridania. That will give you a feel for it and then you can decide if you want to go back to Arcanist to unlock Scholar or stick with Conjurer for White Mage (which requires level 15 Arcanist, so those levels are a 'waste').

[Highlanderx]

That will give you a feel for it and then you can decide if you want to go back to Arcanist to unlock Scholar or stick with Conjurer for White Mage (which requires level 15 Arcanist, so those levels are a 'waste').

Leveling sub-classes are never a Wasteyou gain access to more cross-class skills, Some of which if you don't have while doing end-game content your gimping yourself and the party. And most class require a higher level than 30 to learn all their skills available for cross-class.