How do I pass the nid of the current node to the first step multistep modal form?
I think the simplest way would be to change your path to incorporate the nid, e.g.

function MYMODULE_menu() {
  $items['mymodule/%ctools_js/modals/%node'] = array(
    'title' =&gt; 'Title',
    'page callback' =&gt; 'MYMODULE_callback',
    'page arguments' =&gt; array(1, 3),
    ...
  );

  return $items;
}


Change the path in the link

$build['links']['popup']['#links']['mylink'] = array(
  'title' =&gt; 'show popup',
  'href' =&gt; 'mymodule/nojs/modals/' . $nid,
  'html' =&gt; TRUE,
  'attributes' =&gt; array(
    'class' =&gt; array(
      'ctools-use-modal', 
      'ctools-modal-mymodule-style',
    ),
  ),
);


And in your callback:

function MYMODULE_callback($js, $node) {
  // $node is fully loaded, continue as normal...
}