Show all parts in multipage post

Well, you can turn it off completely or use the following code along with some sort of conditional statement to switch it on or off.

The multipage part is set up in the setup_postdata() function in wp-includes/query.php. There’s an action at the end of the function called the_post.

If you hook onto that and then modify the globals that are set up in the function you can undo the multipage config:

add_action( 'the_post', function( $post ) {
    global $pages, $numpages, $multipage;
    $pages = array( implode( '', $pages ) );
    $numpages = 1;
    $multipage = 0;
} );

Add that to your functions.php or plugin and you’re good to go.