[Zubersoft]

1. If Settings->Storage->Let MobileSheets Manage my Files is checked, and you are importing from a folder not under the storage location, then no, you do not need to keep the original file. You can delete it if desired. If you have unchecked the "Let MobileSheets Manage my Files" setting, then you cannot delete the original, as MobileSheets will be pointing to the file path of that file.
2. This has almost no impact to your storage - it's just adding some data in the database, which is just a few bytes.

Mike