There is absolutely no way he would have been allowed to head to Europe. Most scientists weren't even allowed off the complex at Los Alamos since no one was supposed to know where they were. (I regret that this means no stories about Howard and Oppenheimer getting into horrific fights. "Oppenheimer was a jerk with an enormous ego" was the main thing I heard about Los Alamos for most of my childhood and MCU Howard doesn't seem particularly good at putting up with that sort of thing.)