Do we have any wikis on missions and which ones you need to qualify for mission X or Y?
Do this mission and then you'll get that mission, and so on?

And if they aren't trees, what's the path for the mission lines?

I've heard rumors that there is a mission where if I complete it, I can get extra NPC slots. I would like to focus my missions to be able to get that quickly, if possible...

They are not rumors. 3 Mission chains give you 2 npc slots for each one ending in 8 total, First is Zolazin Chain then Lazuli then Shadin

Thanks. It's kind of hard to parse out that table for missions. Thanks, and I'll go digging for those missions to try and get some more NPC slots...