Its been a few years since I've done this personally but I'm pretty certain you can change your PIN at check in. You can have different pins for different people in the party so it should not be an issue to do this. Just a thought... are you looking for each adult to have their own PIN? If having the same PIN isn't an issue you could just pick a generic PIN (like a date of a holiday or something similar) and you could both use it. Then you wouldn't have to change anything.
Yes you can change it at check-in or just not set it at all, and when they tap their magic band at the check-in desk, the CM willl ask him/her to put in a pin number in the reader.
Yes you can do this. We did it in April. I did online check in and it forced me to assign a PIN # to our adult friend who was coming with us. (I also had to assign my credit card to his band as well.) When we got to the hotel we were able to change his band to have his credit card on it as well as change his PIN #. It was no problem and easy!
Yes. We did this at check-in last year with another family that was under our reservation. They set it to charge under their credit card with their own pin. It was easy.