Mary Poppins is just a loving woman, who teaches the Banks children about the world and more importantly, the people in it, and the supernatural events are seen through the children's eyes.
The park scene (Jolly Holiday) is about using your imagination
Supercalifragilisticexpialidocious is about language and using it to your best advantage (all words were made up at one point), and it's also whimsical, of course.
She's helping the children develop a moral center, which is why Temper, Temper is so important, the children are creating the magic there, but it's really their conscience that they are wrestling with in that scene, but it manifests itself in a Poppinsesque way (read: magical) because that's how the children see Mary as presenting these ideas.
Only the most important lesson is told without magic (feed the birds), the lesson about treating people fairly and not judging them, its a lesson that all the characters learn in the course of the piece.
Its a modern (relatively) fairy tale.
And if you don't buy that, it's just a story. She is neither a witch nor a ghost, speculating on that is missing the point and it doesn't matter one way or another.
I too think the musical is truly a special one, and Disney aside, were it made by anyone else (in this incarnation) more people would be flocking to its virtues, but many people equate Disney with fluff and the bucks, which I used to as well (I hate Lion King) but this one is based on a good story and has good music (even the new music).
I saw it planning on hating it, but I adore it. It has a good message and I applaud it.
"Fenchurch is correct, as usual."
-Keen on Kean
"Fenchurch is correct, as usual."
- muscle23ftl