The main difference is that as Asset is an abstraction to contain information that a file could not contain. For example a file only knows about itself, where as an Asset can contain information of multiple copies of the same file, the location of each, plus an overall understanding of the file in metadata.
All files can be uploaded, stored, downloaded with iconik, and certain file types have support for previews. A full list is available at the Support File Types page
Yes, you can do this by adding all the assets that you wish to update to a collection and using the Bulk Metadata Update function.
It's on the current roadmap to introduce this feature.