Here's how it goes:
Santa is magical and doesn't need to stop at each individual house one after the other. He actually creates duplicates of himself and they are what travels to each house. Each house has it's own Santa, and that one is customized for the household.
So, if you have a house in the United States, Santa is dressed in the familiar red with white trim (thank you Coca Cola). If the house in in the United Kingdom, then Santa looks like Father Christmas. Along these same lines, if the house has a white family, then the Santa is white. If it's a black family, then Santa is black, and so on.
No, the guys in the malls, town halls, museums, or where-ever you go to meet Santa aren't really him, but they are, in fact, his helpers. These men (and women, where Mrs. Claus is around) have a direct line to Santa and they tell him what each child said to them. So, while the child is not really talking to Santa, Santa will know what was said.
Even though he doesn't require a chimney to enter a house (remember, he's magic), Santa does need permission. This permission can come in several forms. The one our family uses is something called a "Santa Key" that we leave outside on the door knob. Santa uses this to enter the house and he then hangs it on the tree.
So, there you have it. My unified theory of Santa Claus that, I think, helps explain some of the "unexplainable" things about him. Feel free to adopt or adapt this for your own use, and please let me know how it works for you.