One of the reasons I like the approach to FEA is that it's one recent attraction in which they haven't tied themselves in knots trying to develop a linear story that makes literal sense at the expense of giving people what they want in a fun ride. In this case, you can fill in your own blanks as to why she was in the ice palace: Who knows, maybe she just likes to go up there to sing by herself sometimes? I don't think the ride would have been any better if they showed her in a different palace without the images from the film singing the same song and certainly not if they cut the song altogether because that already happened in the film and she would be in a different state of mind now.
It works, I think, precisely because it's just a ride through the world of the film with just the loosest of stories to tie it all together and that allows them the best of all worlds by being able to show all the things people liked in the film without literally having to recreate its plot.
What worries me from what we've heard about Tiana is that the story is coming at the expensive of making the most fun and engaging ride they can.