Please take a look at your plugin. It fails to include its templates correctly if the wp-content directory is not in the same directory as site_url. It is a perfectly valid WordPress install if WordPress is in a sub-directory from document_root and wp-content is also a sub-directory in document root.
Your plugin also breaks if the name of your plugin folder is changed - take a look at the plugin_url function.
Similarly your templates will fail since they attempt to climb the directory structure to find wp-load.php. I suggest you look at writing a hidden div on wp-footer and using the standard WordPress constants to find your files and values.
Try making your plugin work in this kind of structure:
wordpress (directory with WordPress codebase) index.php wp-config.php wp-content