It makes perfect sense for Buddy to become a children's book author. He understands the mind of a child, has an active imagination, is good at connecting with others, and firmly sticks to the morals he encourages.
Discussing children's book ideas, Miles Finch speaks unfavorably of rural settings and expresses his belief that a protagonist shouldn't be too vulnerable (kids are vulnerable enough as it is). The children's book at the end, as well as the movie itself, seems to take his advice: it takes place in New York City and features a protagonist who, while childlike, can do all the things a grown man can do.
The Gimbels manager is not a bad guy, given he is Nice to the Waiter with his employees when not under pressure. Thus, it makes sense that he would allow Buddy as a kids' author to read aloud at the store a few years later. It was water under the bridge, and it's implied he found out that this guy was the real deal.
When you think about it, Buddy isn't the only manchild in the movie. The Gimbels manager keeps arguing back and forth with Buddy about minor things, like whether or not there's singing at the North Pole. Miles Finch gets violent over (what he thinks is) a petty insult. Walter's boss refuses to reschedule his meeting, then yells and fires Walter on the spot when Walter leaves to save Buddy. And there may be others. Buddy's childish behavior isn't that different from these other characters, it's just that Buddy keeps a childlike sense of wonder and kindness, while these other characters hold onto more negative childlike qualities while adopting a more cynical view of the world.
As stated below, Buddy should not be able to survive on the elf lifestyle. However, if his night in Gimbels and the snowball fight are any indication, the man is clearly not a normal human. It's likely that growing up surrounded by elf magic did give him some elf qualities after all, just not to the degree of an actual elf. This is what allowed him to survive thirty years of sugar and 40-minute nights.
Also demonstrated when he decorates Gimbels: even if Buddy works slower than the average elf, he is still quite tall, and this means he can compensate by not needing a ladder to decorate from the ceilings/rafters. And having come from a place where he's had to constantly watch his step due to a size class mismatch, in a human-sized environment he's light-footed enough to casually and deliberately avoid security if he so chooses.
The beginning of the movie shows Buddy in an orphanage as a baby, where a nun is putting him in his crib. She tucks him in and says she hope he will be adopted, soothing him. Then Santa comes in and Buddy crawls into his bag for a teddy bear, where he is accidentally brought to the North Pole. The movie doesn't address this, but just imagine how devastated that nun was when she woke up the next morning and realized Buddy was missing. She most likely never found out what happened to him. For all she knows, Buddy could've been kidnapped.
Let's just hope Santa left her a note or something explaining what happened to Buddy after he found him. It would at least be sensible to let her know Buddy was okay and that he found a home.
At the beginning of the movie, Papa Elf mentions that Santa's sleigh struggles to run due to humans lacking Christmas spirit. And when does it fail completely? When Buddy is at his lowest point, from his father blowing up at him. It's very possible that Buddy's own sheer spirit and cheer were among the only reasons why the sleigh had been able to run at all after a certain point in time, possibly for the past several years, which begs the questions: what would have happened all those years if Buddy had never crawled into Santa's sleigh and been raised as an elf? How soon would Santa's sleigh have gone out of commission? And what would happen to the world once it had?
What would Buddy have done if he hadn't seen Santa falling out of the sky? From the way he was looking over the bridge, he might have considered jumping.
What would the Central Park Rangers have done to Santa Claus, given how Laura the journalist said she was against the decision to mobilize them? That is, if they hadn't realized the "hoax" was just a man in a red suit and grounded reindeer.
Buddy's first interaction in the real world is getting mauled by a raccoon... which only come out during the day if they're rabid. Buddy is extremely lucky he didn't get infected with anything.
Buddy is a human man raised by elves, who didn't seem to make very many modifications to their lifestyle for him based on his human physiology. Buddy has clearly adapted to eating a 100% sugar diet and only getting 40 minutes of sleep per night, but that is not how humans are supposed to live. His personality and behavior—shown to be unusual by both human and elf standards—may well be the result of brain damage from 30-plus years of severe sleep deprivation and malnourishment.
He is also held to an elf's significantly longer development timeline, making him roughly elementary-school aged in his late 30s. Papa Elf is at least 490 years old (if Buddy's drunken comments can be considered accurate), and even that's not too old (Bob Newhart was 74 when the movie was released). Even by the most optimistic human life expectancy, Buddy was going to die at an extremely young age compared to his elf family and peers and Papa Elf will likely outlive his adopted son.
