Conditionally insert content in wp_nav_menu using custom walker

I am trying to dynamically add a nested ul to a list item in a wp nav menu when the walker encounters its own item $id that matches an id passed to the walker.

I created a custom walker which is required by my functions.php file. I am calling the walker inside of a sidebar script in its own sidebar.php file. My sidebar.php has this code:

if(is_singular('event')) {
				 $event=	em_get_event($id,'post_id');
				 echo 'single' .$event->post_id;

			global $post;
			if($post->ID == $event->post_id) echo 'a match!';

			if(in_array($post->ID, $em_past_exhibit_post_ids)) {
				$nav_id = 117;
			} else if(in_array($post->ID, $em_current_exhibit_post_ids)) {
				$nav_id = 115;
			} else if(in_array($post->ID, $em_future_exhibit_post_ids)) {
				$nav_id = 224;
			} else if(in_array($post->ID, $em_past_concert_post_ids)) {
				$nav_id = 562;
			} else if(in_array($post->ID, $em_current_concert_post_ids)) {
				$nav_id = 558;
			} else if(in_array($post->ID, $em_future_concert_post_ids)) {
				$nav_id = 560;
			} else {
				$nav_id = 61;
			} 

			$ptitle = get_the_title( $nav_id );
			$plink = get_the_permalink( $nav_id ); 

			echo 'nav id is: ' . $nav_id;

			$defaults = array(
							'theme_location'  => 'my-header-menu',
							'menu'            => '',
							'container'       => '',
							'container_class' => '',
							'container_id'    => '',
							'menu_class'	=> '',
							'menu_id'         => 'my-events',
							'echo'            => true,
							'fallback_cb'     => 'wp_page_menu',
							'before'          => '',
							'after'           => '',
							'link_before'     => '',
							'link_after'      => '',
							'items_wrap'      => '<ul id="%1$s" class="%2$s">%3$s</ul>',
							'depth'           => 3,
							'walker'          => new my_event_walker,
							'walker_nav_id' => '61',
							'walker_el_name' => 'test'
						);

						function modify_nav_menu_args( $args )
{
	if( 'my-header-menu' == $args['theme_location'] )
	{
		global $post;
		$id = $post->ID;
		$args['link_after'] = 'id is:' . $id;
	}

	return $args;
}

add_filter( 'wp_nav_menu_args', 'modify_nav_menu_args' );

						wp_nav_menu( $defaults );

In my actual walker, in the start_el function, where output is structured, I thought I could add (at approx. line 141 of just the Walker Nav Menu class ) `$item_output = $args->before;
$item_output .= ‘<a’. $attributes .’>’;
/** This filter is documented in wp-includes/post-template.php */
$item_output .= $args->link_before . apply_filters( ‘the_title’, $item->title, $item->ID ).$args->link_after;

if($item->ID = $args->walker_nav_id) {
$item_output .= ‘<ul><li>’ . $args->walker_el_name. ‘</li></ul>’;
}
$item_output .= ‘</a>’;

$item_output .= $args->after;
`
the site is only local right now. But I’m wondering how I would a. pass the $nav_id to the walker class and b. how and where I would access the item ids that walker is ‘walking’ to create the menu so that I can compare the two. Do I need to pass that as an argument to link_after or shouldn’t I be able to modify the walker directly? Thank you for your thoughts.