NPC level determination?

Hi

There are a couple of things I'm not certain about, maybe
you can help me with it.
During my first through of game I remembered that I didnt
have Viconia with me in the beginning, and when I picked her up in Chapter 6
I saw her level was 13, that was pretty high compared with other npc's.
So just for my curiosity I picked up another npc's who I dichted before
I went to asylum. And I found out they were in same level as before.
Second time I played the game I had Viconia with me, and she was at level 8,
so my question is: Did Viconia adventure alone and get her XP's or what?
maybe it was getting too boring for her just standing in the Graveyard District all the time I guess.
I think the npc's level will raise according to the game progression if you didnt
pick them up first time you met them. Any comment?
BTW, does dual- or multiclassed mage character have one additional spell in every spell levels, or it
depends on something else, because my swashbuckler/mage does?

Thanks for reading.

NPC levels will be determined as soon as you add them to your party, based on the protagonist's own level. However, an NPC you take into your party will never be below level seven, and never above level thirteen. Once you take an NPC into your party, if you drop them, then come back later, their level will still be the same. It's as simple as that.

Thanks, that's what I thought.