I'm a little confused myself now. I know they've changed their minds at least once about how children would work and a few other things that effect the family system and I'm not sure if that was before or after the design journal. My understanding was that the 1 real world week = 1 Elyrian year would apply to everyone, but the design journal almost makes it sound like children will grow faster than adults.
Question 1: My understanding is that you will be able to take over a child between the ages of 15 and 18 automatically, and beyond that will cost some story points. This seems a pretty narrow window to me, but we don't know how difficult story points will be to gain or how much it will cost to take over a child a few years older than 18.
Questions 2 and 3: The Blog Post "Down the Rabbit Hole" discussed the changes they made to the heir system. You will be able to choose as your heir any NPC in your dynasty who is open or who you have the code for. You will need story points if they are older than 18, and the NPC cannot be older than 52, but you can pick a sibling, niece, nephew, child, grandchild, or even a parent if you die particularly young.
The "Down the Rabbit Hole" journal said that you can only lock your immediate descendants (and parents if they are NPC's), but you can pick extended family members as heirs if they are open. I think I heard somewhere that they were considering letting you pick an primary and secondary heir to lock from any of your family members, but I'm not sure if that's actually true.