Jen you should do the back to back!
I hate to discuss costs and all cuz it is kind of bad manners, but we added the 3 day for 3 people for about 900 bucks including taxes fees and
travel insurance. I think it was $350/adult and $150 for the peanut, so for you with your mom it should be about the same, $900 more (depending on room of course)....and it'll be less hectic than the park (although the park is definitely great!)
I found out the basics of having to do the back to back. You pack your stuff and put it out in the hall like everyone else. If you are staying in the same room, they will leave it there. If you are moving to a different room, the front desk will store it for you and have it taken to your new room.
You have to be off the ship at 9am and go back out to the terminal area, but can come back around 10:30. Because you'll already be familiar with the ship and all, it will basically be a 'day at sea' without being at sea. You get another awesome day of parties and tours and get to do all the things you didn't have time to or missed on the first one.

but you do have to do the icky practice firedrill thing with the life vests...I don't enjoy that part.