If I set a release date to some time in the future, will my music be hidden until then?

No, your release date does not affect visibility. When you first create an album it is hidden from the public as a Draft, and will remain so until you are ready to release it (by clicking “Publish” in the album editor). If at any time you’d like to hide your album from public view after it has been released, select the “Private” option, then click “Update.”