Question on classes/jobsFollow

Summoner and Scholar are both jobs that are unlocked through their base class, Arcanist. So you have to level Arcanist (not Scholar) to 30 and some other yet to be determined job to 15.

A job is like a specialist mode for playing the base class. So your Summoner and Arcanist share the same level. You Summoner and Arcanist (and Scholar if you choose to unlock it) will level at the same time.

You don't need to further level the level 15 job that was required to unlock Summoner. However, leveling it will give you access to abilities that you can cross class when you play Arcanist and Summoner. Summoner will have access to all of Arcanist's abilities and will be able to cross class abilities from two other classes (the one that you had to level to 15 to unlock Summoner and one other class). So there's also a 3rd class that's not required in unlocking Summoner, but could be useful to level so that you can cross class its abilities.

Arcanist, Summoner and Scholar will also share bonus attribute points.

Correct. Summoner is a specialty of your Arcanist job, so any level you reach with Summoner, your Arcanist will automatically be should you switch back and vice versa.

Maybe some of the new classes will not be advanced jobs at all, like Archanist.

Could be that Dark Knight will be a base class. Just a possibility.