Did composer Hans Zimmer rip off Edith Piaf? He tells the NYT how he used "Je Ne Regrette Rien" for his main theme. (YouTube video comparison is below.)
What is Inception really about? It's pretty obvious that you can apply it as a metaphor for making movies. This blogger argues that Inception is really about director Chris Nolan's anxiety about delivering a blockbuster.
What happens at the end? Does the top topple? Is it all a dream? Only Inception costume designer Jeffrey Kurland knows for sure. He reveals key info in this interview in Costumes on Film:
JK: Costume design reflects greatly on the movement of the plot, most significantly through character development. Character development is at the forefront of costume design. The characters move the story along and with the director and the actor the costume designer helps to set the film’s emotional tone in a visual way. In a more physical sense the costumes’ style and color help to keep the story on track, keeping a check on time and place.
On to the second part of your question, the children’s clothing is different in the final scene… look again…
UPDATE: Apparently, Nolan got his idea from a Donald Duck comic.
Here's the Zimmer music comparison:
In this College Humor video, the Inception team gets confused about what they are doing.