Have done it many times. There's no trick to it. Just tell the CM checking you in you need to add someone. You'll just have to pay the extra adult fee per night if you're over 2 adults in the room.
Yes, you can do this. No trick to it as long as you are still within room capacity. The new person will not have any package add-ons (tickets, dining, etc.) and all they will have is a key. If they kick in the third adult charge you may end up paying that.
Kids aren't considered adults at Disney until they are 18.
The only caveat about adding someone at check-in is if they're flying in, they won't be eligible for Magical Express until they are added onto the reservation.
No additional cost to add a 17 yo. As the pp said, Disney doesn't consider kids as adults until age 18, so no extra cost unless there are more than 2 people age 18+ in the same room.