ShellExecute("Fiddler") no longer works because Fiddler is no longer registered in the APP PATHS registry key. This breaks a 14-year-old mechanism of launching Fiddler.
Incidentally, It appears that Fiddler now installs on a per-user basis /by default/? (That's not great, as this will break extension installers, it is less secure, etc).
Also applies to the FiddlerScriptEditor, apparently.