If I read the right thing (there are also bits related to adoption events generation when you don't have any heirs, not related to the schemer ability) and read it correctly, assuming you can adopt, ie your leader is adult, alive, etc:
to be eligible for adoption, a character must be :
- not dead
- not your spouse
- not yourself
- not a descendant (can't adopt your own grandchildren)
- younger than the adopter
- not a parent (a character with a child or more isn't eligible for adoption)
So typically, young nieces and nephews, younger courtiers, etc can be adopted.
How could you do that in a single turn?
Ok, so I'm answering about 2 weeks late, I have to apologize.
Story time !
So, this was during Early access, January 2021(by screenshot dates), and I was exploring One-City-Challenge gameplay at "The Great" difficulty, which I was still finding pretty tough (I even wrote a sort of guide for it which should still be around here somewhere, that was possibly my first post on this forum).
The game was different in those times. Tutoring wasn't a thing, sovereignty was a T2 tech, specialists had no scaling costs, etc.
One of the game mechanics I was exploring (reverse engineering through testing, I should have just peaked at the code) was territory expansion and the rules to make minor cities etc, which are a key source of victory points in OCC.
Grabbing tiles normally requires you to make urban improvements/tiles. Even with dedication it is usually too slow to get urban sprawl beyond the first tier of city site neighbouring your capital and only city - even with a "Builder" archetype leader which you tend to have and cherish in OCC.
You can also make rural specialists, but in OCC as I was playing it then, each citizen was more precious as a urban specialist that can be further upgraded. Buying tiles with colonies is strong but the price is scaling very quickly when you aim to make multiple minor cities.
Forts were made urban in a patch and as you can build forts anywhere pretty quickly (one worker turn, two on a hill), you could pepper the map with forts for a low stone and worker turn cost.
If you had good understanding on how territory expansion chains, you could use that to prepare a land grab, then buying a single tile or training a single specialists and engulf an incredible swath of land for the price of a tile.
Luckily, I still saved screenshots at that time so here is what the preparation could look like (T93 from the screenshot name):
and what the result was with a single additional tile grabbed by a completed gardener (T94 screenshot):
I think I counted the tiles at the time, and it was around a hundred. This was my first try just after discovering this, of course I got better at exploiting it while completing that game (while reporting the problem to the dev team ! honest !

).
For the sake of silly memories, here is a screenshot of the same game on T171, when I claimed victory. Please check the minimap : My single city was covering about one third of the world
And that, dear civilization fanatics of the World of Old, is why forts are not urban tiles these days.
/story time.