Add parent menu item in sub-menu in custom nav_walker

Currently I'm working on a project where we need to append the parent title as first item in the submenu for a mobile menu.

What I try to achieve is this:

 ul class="menu"
   li
     a href="linktomenu"Level 0 item/a
     ul class="submenu"
       lia href="linktoparent"Show All Level 0 item /a/li
       lia href="#0"Back to parent/a/li
       lia href="linktomenu"Level 1 item/a/li
       lia href="linktomenu"Level 1 item/a/li
     /ul
   /li
 /ul

This is the code I currently have

 ?php

class mobilenav_walker_nav_menu extends Walker_Nav_Menu {

  private $color_idx = 0;

  // add classes to ul sub-menus
  function start_lvl( $output, $depth = 0, $args = Array()) {
    // depth dependent classes
    $indent = ( $depth  0  ? str_repeat( "\t", $depth ) : '' ); // code indent
    $display_depth = ( $depth + 1); // because it counts the first submenu as 0
    $classes = array(
                'sidebar-menu',
                ( $display_depth % 2  ? 'menu-odd' : 'menu-even' ),
                ( $display_depth =2 ? 'sub-sub-menu' : '' ),
                'level-' . $display_depth
              );
    $class_names = implode( ' ', $classes );

    $back_btn = '';

    if($display_depth  0){
      $parent_label = 'li class="go-back"a href="#0"span class="parent_label"/spani class="fa fa-close"/i/a/li';

      $back_btn = 'li class="see-all"a class="hit-area" href="#"Back/ai class="fa fa-chevron-left"/i/li';
    }

    // build html
    $incri = $this-color_idx-1;
    $output .= "\n" . $indent . 'ul class="menu__level" data-menu="submenu-'.$incri.'"' .$parent_label.$back_btn. "\n";
  }

  // add main/sub classes to li's and links
  function start_el( $output, $item, $depth = 0, $args = Array(), $id = 0 ) {

    global $wp_query;
    $indent = ( $depth  0 ? str_repeat( "\t", $depth ) : '' ); // code indent

    // depth dependent classes
    $depth_classes = array(
        'sidebar-item',
        ( $depth == 0 ? 'li0' : '' ),
        ( $depth == 1 ? 'li1' : '' ),
        ( $depth == 2 ? 'li2' : '' ),
        ( $depth == 3 ? 'li3' : '' ),
        ( $depth % 2 ? 'menu-item-odd' : 'menu-item-even' ),
        'menu-item-depth-' . $depth
    );

    if(in_array('menu-item-has-children',$item-classes)){
      $has_children = 'data-has-children="1"';
      $has_children_var = 1;
      $depth_classes[] = 'has-children';
    } else{
      $has_children_var = 0;
      $depth_classes[] = 'no-children';
    }

    $li_attributes = 'data-level="'.$depth.'" data-title="'.apply_filters( 'the_title', $item-title, $item-ID ).'" '.$has_children;

    $depth_class_names = esc_attr( implode( ' ', $depth_classes ) );

    // passed classes
    $classes = empty( $item-classes ) ? array() : (array) $item-classes;
    $class_names = esc_attr( implode( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) ) );

    // build html
    $output .= $indent . 'li class="menu__item"'.$li_attributes.'';

    // link attributes
    $attributes  = ! empty( $item-attr_title ) ? ' title="'  . esc_attr( $item-attr_title ) .'"' : '';
    $attributes .= ! empty( $item-target )     ? ' target="' . esc_attr( $item-target     ) .'"' : '';
    $attributes .= ! empty( $item-xfn )        ? ' rel="'    . esc_attr( $item-xfn        ) .'"' : '';
    if($has_children_var == 1){
      $attributes .= ' data-submenu="submenu-'.$this-color_idx.'"';
    }
    $attributes .= ' class="menu__link"';

    $item_output = sprintf( '%1$sa%2$s%3$s%4$s%5$s/a%6$s',
      $args-before,
      $attributes,
      $args-link_before,
      apply_filters( 'the_title', $item-title, $item-ID ),
      ($has_children_var == 1 ? $args-link_after : ''),
      $args-after
    );

    // build html
    $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );

    if($has_children_var == 1){
      $this-color_idx++;
    }
  }
}

This is the current output I have

  ul id="menu-hoofdnavigatie" class="sidebar-menu level-0"
    li class="menu__item" data-level="0" data-title="Menu Item Level 0" data-has-children="1"a data-submenu="submenu-0" class="menu__link"Menu Item Level 0/a
      ul class="menu__level" data-menu="submenu-0"
        li class="go-back"a href="#0"span class="parent_label"/spani class="fa fa-close"/i/a/li
        li class="see-all"a class="hit-area" href="#"Back/ai class="fa fa-chevron-left"/i/li
        li class="menu__item" data-level="1" data-title="Menu Item 1 Level 1"a class="menu__link"Menu Item 1 Level 1/a/li
        li class="menu__item" data-level="1" data-title="Menu Item 2 Level 1"a class="menu__link"Menu Item 2 Level 1/a/li
        li class="menu__item" data-level="1" data-title="Menu Item 3 Level 1"a class="menu__link"Menu Item 3 Level 1/a/li
        li class="menu__item" data-level="1" data-title="Menu Item 4 Level 1"a class="menu__link"Menu Item 4 Level 1/a/li
        li class="menu__item" data-level="1" data-title="Menu Item 5 Level 1"a class="menu__link"Menu Item 5 Level 1/a/li
      /ul
    /li
  /ul

I'm quite new to the wordpress navwalker, can you help me on this one? I've searched the entire internet, but could not find a solution for my problem so far.