If the association instructs Windows to use DDE to open the document, no application is run by the association, so there is no application to wait for. If this is the issue, you might have better luck if you give the run the executable directly instead of just specifying the path to the document.

Even then, it might never work as you wish when Excel is already running. Some applications check if there's another process running the same program, instruct that process to open the document, then exit. It seems likely to me that Excel is such an application.