Update, I got it working last night but only by running the exe directly out of the 4.5 engine folder - the same problem still persists for any 4.5 project or 4.5 engine in the launcher. Is it possibly related to windows 8.1 permissions somehow?
Could the launcher not have permission to launch the 4.5 engine? It still works with 4.4 projects though, so that seems really odd.