After node creation how do i add simple buttons to edit the field values
The $node_id is blank as there is no value passed for it. In order to get node_id in the submit handler function, you should pass the node_id in the form. So pass the node id value as a hidden field in the form and get the value of hidden form element through $form_state of redirect_button_form_submit. In that way you can do it.
Example :
$form['node_id'] = array('#type' => 'hidden', '#value' => $nid);
Yes it is possible for you to edit $node->content['apikey'] if and only if it is saved in database. Create a new field as apikey in the content type and store it.
Hope it helps. Thanks.