However, there's a flaw in her belief. Next year, when we take her to see her favorite, or "real" Santa, he is sure to look different, since they get a different person to play him every year, right? That brings me to the question; How do I explain the different Santa's every year?
You explain that when she asks about it. Looks like you've got a year to not worry about it.
Maybe they don't get a new Santa at the White House every year. Maybe the same person has been playing Santa for a long time. If there is a different Santa there, how do you know she'll remember enough about his face from the age of 5 and 6 to say "this isn't the same person"? From what you said, it looks like the "proof" to her is more about the fact there was a workshop there.
The reason there is a flaw in her belief is because she (right now) believe in something that isn't real. It's inherently flawed because it involves telling her untruths to get her to keep believing.
If there were proof, it wouldn't be called "believing."
Are you saying the word believe means there is no proof?
I believe the planets in this solar system revolve around the sun, and that water and wind erosion formed the Grand Canyon. I believe water droplets in the air, and the sun shining on them at just the right angle, compared to where I'm standing, form the beautiful colors of a rainbow.