A little out of order, but: for the 2 bed request (it is 2 queens at YC, not doubles btw), call the reservation line & add that as a request.
Did you customize your magic bands? If you did, you should get them in the mail. If you declined new bands, just bring your old ones. They will work fine as long as you did not deactivate them. If you did, reactivate them before you go. If you didn't customize or decline, they will have the standard grey ones waiting for you when you check in.
The PIN will have to be reset at the lobby desk. I don't know of anyway you can do it over the phone. Although if I am wrong, I'm sure someone will correct me & tell you how to do it.
If you do online check in, you get an option to receive texts from Disney. This is how you find out if your room is ready & lets you go directly to your room if it is. And if you get the "room ready text" your magic bands (new or old) will let you in the room. So this is a big benefit of online check in. You may not need to go to the check in desk at all. But in your case, sounds like you will have to stop there at some point to reset your PIN. As a reminder, you need that PIN if you intend to charge anything to your room, at shops or restaurants & to authorize any dining plan use. So you will need to stop there pretty early on your trip anyway.
So the advantage to online check in is that you may be able to go directly to your room. But even if you don't, it does make the check in process quicker, because they have all your information already in the system.