Short of using a TA, I would contact Princess and get with a CVP (Cruise Vacation Planner). I bet you ended up with the overseas call center where you can end up with misinformation. 21 is the drinking age for most voyages.
It might help to have a letter from the other teen's parents that they are authorized to travel with you. However, it really is no big deal to swap cabins on board.
There is also no issue in ordering Plus Fare package on board day 1. Besides, any OBC you have goes towards paying for it and you will get $10 rebated for not having Medallion mailed. Of course, make sure the people are swapped in the System before ordering.
You can also order the Medallion Net for teens, but I would simply upgrade your include package to a 4-device package and then the teens can login. Everyone will always have one connection and if anyone has a second device, you just bump off the original.