>But then, just to make life more confusing still, any existing work that would have it's copyright shortened to May 1, 2032 or sooner has an extra change: anybody who bought a license to such a work loses half the length of the license.

I think it's kind of the other way around. If the the law would cause copyright to expire while their license is still active, the copyright is extended for a bit, so that the person holding the license gets to keep the value of that license for a bit longer.