How can I modify the displayed discount message?
Documentation isn't complete but is available here.  It specifically mentioned control over the location as something that doesn't exist yet.  If you're JavaScipt-savvy, a single line of jQuery will let you move the box wherever you want; a second line will let you control the wording.  I wish there was an answer for non-coders to do what you're asking for.  If this is important to you and you're not a coder, you might be able to fund someone (either the core team or another CiviCRM developer) to add those features.