Well, if you were a customs agent, and Minnie Mouse was standing before you and handed you a birth certificate for Tinker Bell, what would you think?
You've got a real problem there. Do you have any documentation to prove that Minnie Mouse and Tinker Bell are the same person? On your birth certificate, what does it say for your mother's maiden name? Her legal name, or her made up name? Does she receive social security checks or benefits? Who does the statement/checks come payable to? A social security card with her given name, and something else with the same social security number and her made up name? Income tax returns? What name did she use for her marriage license? You have to have something that shows the transition from one to the other. Even so, that in itself probably won't do it.
Pre-9/11, we sailed with Disney. I forgot my wife's birth certificate. They said no less than 5 times on the bus from the airport "if you don't have proof of citizenship, you cannot go on the cruise". On the bus ride to the resort, we were discussing how we could call home and have someone find it and then FedEx it down to FL. When we arrived at the resort (we did the cruise after the resort), the folks there were very understanding and said it happens all the time. They had an affidavit of citizenship form (or something like that) which my wife filled out, signed, showed a driver's license, and that was that. No questions asked, no problems with the cruise at all. Post-9/11 - I have no idea if the rules have changed.
I personally think you shouldn't be asking your question here, because what if someone tells you "it's ok, don't worry about it" and then when you get to the cruise they tell you "no way"? No matter what anyone says here, it isn't going to help.
First you should call DVC and see what they say. Next, regardless of how this situation is resolved, you should get some kind of legal documentation to prove your mom is the same person named on her birth certificate - even if that means legally changing her name.
Personally, what I would do, is take her original birth certificate, her driver's license, marriage license, and something that clearly shows the transition and apply for a passport and pay the extra $60 fee for expedited handling (I think it's 3 business days). If you go to the US Postal website, all the forms are online. If you are lucky, you can blow it right through the passport office, get a passport for your mom, and not have to deal with the problem any longer. But again, I would still make some phone calls and have contingency plans in place.
http://www.usps.com/common/passport/
Good luck with this.
