Make featured image be shareable across multiple pages?

You could easily do this by hooking into the save_post action and updating the featured image custom field. In your functions.php file, add the following code:

function interit_featured_image( $post_id ) {

    // Get Parent ID
    $parent_id = wp_get_post_parent_id( $post_id );

    // Only proceed if this page has a parent
    if ($parent_id != 0) :

        // Get Parent Thumbnail
        $parent_thumbnail = get_post_meta( $parent_id, '_thumbnail_id', true );

        // Get Parent Caption Data (for the other plugin)
        $parent_caption = get_post_meta( $parent_id, '_cc_featured_image_caption', true );

        // Set Thumbnail
        update_post_meta( $post_id, '_thumbnail_id', $parent_thumbnail );

        // Set Caption (again, for the other plugin)
        update_post_meta( $post_id, '_cc_featured_image_caption', $parent_caption );


    endif;

}
add_action( 'save_post', 'interit_featured_image' );