As you've discovered, the only way that you can currently get the U.S. Copyright Office records to reflect different creation dates is to register the works on separate applications. Having an accurate creation date in the copyright registration records might be helpful if someone later tries to say that you copied the songs and there's a dispute about the creation date. (Of course, if there was a formal dispute such as a lawsuit, much more than just the Copyright Office records could be used to prove creation date.)