Parse XML into Wordpress WYSIWYG editor ONLY for Posts

I've run into a little situation that is hopefully possible to solve. My goal is to take an existing XML file from the server, parse it, and then inject it as a list into Wordpress's original WYSIWYG editor so that the site owner has the list readily available when he writes a new post. Right now, I have this code in my wp-admin/edit-form-advanced.php file:

/**
 * Fires after the title field.
 *
 * @since 3.5.0
 *
 * @param WP_Post $post Post object.
 */
do_action( 'edit_form_after_title', $post );

if ( post_type_supports($post_type, 'editor') ) {
?>
<div id="postdivrich" class="postarea<?php if ( $_wp_editor_expand ) { echo ' wp-editor-expand'; } ?>">
<?php

/** LOAD XML FROM SERVER AND PARSE AS UL INTO EACH NEW WP POST **/
    $xml = simplexml_load_file('../my-folder/file.xml');

    $product = "<br/><br/><h2 style='text-align:center; color:#003300;'><u>Products Available Now</u></h2><br/><ul style='text-align:center; list-style:none; color:#003300;'>";
    foreach( $xml as $value ) {
          $product .= "<li>";
          $product .= $value->Description .= " $";
          $product .= $value->Price .= " / ";
          $product .= $value->QtyUnit .= "\n";
          $product .= "</li>";
         };
?>


<?php wp_editor($product, 'content', array(
    '_content_editor_dfw' => $_content_editor_dfw,
    'drag_drop_upload' => true,
    'tabfocus_elements' => 'content-html,save-post',
    'editor_height' => 300,
    'tinymce' => array(
        'resize' => false,
        'wp_autoresize_on' => $_wp_editor_expand,
        'add_unload_trigger' => false,
    ),
) ); ?>

Although it works, this causes a couple issues.

1) It injects the data into every WYSIWYG editor, including pages, which I would like to avoid. The content should only appear in post editors if possible.

2) It causes a pretty serious bug that erases anything but the list whenever that particular admin page is reloaded. I can't save any drafts, or edit posts or pages unless I keep that session open in the browser during the editing process.

Not sure if these issues can be solved, but any and all help is sincerely appreciated!!

Solution

You should never modify WP core files. It's advisable that you update or restore the original files.

What you need can be achieved with this little plugin:

<?php
/**
 * Plugin Name: Default Post Content
 */

add_action( 'load-post-new.php', 'new_post_so_44123076' );

function new_post_so_44123076() {
    # Only load if post type not defined (only occurs for Posts)
    if( isset($_GET['post_type']) ) 
        return;
    add_filter( 'default_content', 'default_content_so_44123076' );  
}
function default_content_so_44123076( $content ) {
    # Build your own custom content
    $content = "My html content.";
    return $content;
}

Make a folder for the plugin, put the code inside a file (custom-content.php) and put the XML on the same folder.

It can be retrieved like this:

$xml = plugins_url( '/file.xml', __FILE__ );