Miles Finch actually was an elf.He is the antithesis to Buddy - an elf who was raised by humans but doesn't know it. This explains all of his wealth, why he is so mean, and why he was so very offended about being called an elf.
Buddy isn't really a man-child....Well, at least not to an elf. Elves seem to age half as quickly as humans or even slower, so Buddy might actually be just as far along mentally and emotionally as actual elves his age. Unfortunately, at some point, Buddy got too big and/or old-looking and was forced to work with much older elves, which is why they treat him like a child.
Buddy really is insaneAll the scenes with Santa or in the North Pole are in his head. His mother wasn't really dead but had abandoned Buddy in a mental institution as a teenager when she realized how crazy her son was. The doctors in the institution didn't know his name because they had found him in the lobby alone so they called him Buddy, and his childlike mind took that as his name. Santa was really the head doctor, and the elves were his assistants. His mother had left the picture of her and Walter in bag that she left with Buddy, so he'd remember her. The doctors, to spare his pain, had taken the picture from him, but one day the janitor Buddy had befriended (Papa Elf) let slip that he had a mother, and decided to say that she was dead so Buddy didn't realized that she had abandoned him. He gave Buddy the picture, and told him about his father, who the institution knew because the administrator had gone to college with him. Buddy, being strong, was able to break out and steals an elf suit from a drunk leaving a Christmas party. His skills at creative building are explained by autism. After his father yells at him, he does go to the bridge, but instead of seeing Santa he falls off the bridge to his death.
Buddy is actually on a mission.The world is split between South Pole Elves, who try and spread evil, and North Pole Elves who spread joy. Buddy was spending his time in New York converting those who had been touched by evil (E.G The guys in the mailroom, Walter, and to a lesser extent, his girlfriend) and managed to end up converting all of New York, or at least defeating the darkness (Miles).
Elf takes place in the same universe as the original version of Miracle on 34th Street.First of all, Santa speaks of having been to New York as if he spent time on the ground, not just passing through to deliver presents. Second, this explains why Gimbels is still around in 2003, rather than having been bought out and eventually liquidated as it was in reality: the events of the film caused the owners of the two stores to become friends, so that when Gimbels ran into the financial trouble that resulted in it being bought out by BATUS and eventually dismantled in reality, Macy's generously helped bail them out.
The Santa in this movie is the Santa from Santa Claus Is Comin' to Town.He kept Buddy because being raised by a bunch of toy makers worked out so well for him.