Thanks for posting. I'm afraid that choosing the placement of the success/error message is not currently an option included with the plugin. Nor is the option to hide the form fields after the subscribe button is pressed. These are both great suggestions and I'd be happy to pass along these feature requests to our developers for future consideration.
As far as managing this current expectation with users, one suggestion would be add content to the subheader of your plugin's signup form (through the plugin's settings) providing instructions to the user that they'll want to look out for that success message or confirmation email, so that they read that even before clicking the subscribe button.