Enchanted/WMG

Narissa originally came from Earth
Like Howell Jenkins before her, she found a portal to a land where magic existed and found that she had the ability to use magic herself. When she sent Nathaniel after her stepson, she told him what he needed to know to get around New York (and probably gave him some money as well).
 * Pip's ancestors came from Earth as well. Why else would he have a Bronxish accent in ANDALASIA?
 * Taking that a step further, Narissa is Janet Majors (nee Weiss).She realized she wanted more from life than just a happy, suburban life with Brad (and had been somewhat broken by Frank). So, during their honeymoon in New York, she left Brad and, wandering around the city, ended up falling down the portal to Andalasia. She seduced and then killed the king, attempted to keep his son from being a threat, and the rest is history.
 * Being from Earth would explain Narissa being so Genre Savvy. She knows the fairy tale tropes because she saw the movies in the real world.

Andalasia is a matriarchy.
This explains why Prince Edward will only take Narissa's throne if he gets married. If Andalasia were a patriarchy, he would already be king.
 * Well, of course. What else could a proper magic land be but a Princessdom?

Pip is Jesus
Well... He does save the day by Heroic Sacrifice.
 * But -- he doesn't die. He's seen at the end promoting a book he's written.
 * Hello? It's a Disney film. Even in "the real world," Disney metaphysics tend to overrule real-world metaphysics...

After the events of the film, Robert quit his job with the law firm to become a certified marriage counselor.
Because after falling in love with Giselle, helping couples to stay together is a much more fulfilling career for him than helping them to split up. And it makes Morgan and Giselle happier, too.
 * It looked like both he and Giselle ran the dress store at the end. Presumably, she made the dresses and he did the business work. But this is good, too.
 * Maybe he did, and Giselle helps the other way around! Not all of Giselle's fairy-tale advice is purely, well, fantastical--it's always nice to remind your significant other (or, heck, just your friends and family) that they are important to you.

Nancy and Giselle were Switched At Birth.
Due to the magical nature of Andalasia, Love At First Sight works there--"destined" couples do have a Red String of Fate, or some other sort of bond, that connects them together. When they meet for the first time, it goes off to signal to them, "This is who you're supposed to be with!"

Nancy was originally born in Andalasia, and her "string" ended up connected to Edward. But she ended up on Earth, and an Earth-baby, Giselle, ended up in her place. (Hey, we don't know where Giselle's parents went.)

Because Giselle grew up in Andalasia, she acquired most of its inherent magic (such as forest friends and magical music); but she didn't have a proper Red String of Fate. The influence of the realm was such that she could feel like she was falling in love at first sight; but as time wore on, the spell would have broken, and she would have found herself increasingly unhappy.

When Nancy and Edward met in the ballroom, Edward was able to reawaken her inherent magic and make her "red string" activate. Giselle found herself more happy in New York, but retained her magic--both because she had spent most of her life using it and because she kept using it (to run her shop).

This takes place inside The Matrix
Andalasia is the first version of The Matrix (everything is beautiful and orderly); New York is in the Matrix where Neo lives. Giselle is a test to see if a person from the First Matrix is compatible with the modern Matrix.

Giselle and the others are from Toontown
Toontown has portals to the real world besides the tunnel in Los Angeles. There's also the golf hole from Space Jam and, evidently, the New York manhole from this movie.

Giselle is meant to directly symbolize the evolution of the Disney Princesses
At the beginning, she is a true-love obsessed girly girl (i.e. Snow White, Cinderella, and Aurora). After she goes to New York for a little while, like about the time she gets the blue dress, she is still very in the clouds but is SLIGHTLY less naive (i.e. Ariel). Later on, though still bright and romantic, she has a bit more of a spirit and can actually get emotions like anger (i.e. Belle, Jasmine). Then at the climax, she takes action and actually engages in combat (i.e. Pocahontas, Mulan). Finally, at the end, she is a hard working business woman (i.e. Tiana)
 * TV Tropes Made of Win Archive analysis!

The movie is a case study of the effects of a Narrativium on reality
Giselle and the rest of the Andalasians come from a world of Pure Narrativium, and they bend reality to fit the story. Why do the animals help clean, even in New York? It fits the story. A spontaneous choreographed song breaks out in Central Park. It fits the story. Prince Edward being run over by the bikes was an attempt by reality to reassert control, but it didn't work -- that event fit the greater story of Giselle finding her true love.

Nancy's phone had great reception because it magically bypassed the tower system to automatically contact the phone she is calling or the call redirection system that is forwarding to her.
So, erm, yeah. The actual reception isn't really any better than a sewer under Time Square should have.

The "No happily ever afters" thing is perfectly true, in-story, just not in the sense Narissa might have been thinking.
It is a result of that place being 'the real world', not a story - the only true 'end' is death (which might have an Happily Ever After, but so far that would be someplace else), so while people can be happy for long, long times, technically they are not happy ever after.