Problem with logging in WP users automatically

In the end, wp_signon() was the solution, as the cookie was apparently not set with the code I was using.
I followed this solution
https://gist.github.com/iandunn/8162246
to circumvent a need for password and added these functions to functions.php.

I also moved the autologin code from ‘init’ to template where it executes before any headers are sent or any html is output (above wp_head() ).

So now it looks like this:

if (!is_user_logged_in()) :
    if (isset($_GET['user_redirect'])) :
    $user_login_string = intval($_GET['user_redirect']);
    /* Fancy user id retrieval stuff */ 
    $user_data = get_userdata($decrypted_user_id);
    if ($user_data) :
      $user_login = $user_data->user_login;
      $loggedin = programmatic_login( $user_login ); // <--- Gist function
      if ($loggedin) :
        wp_redirect('/userpage');
        exit();
      endif;
    endif;
  else :
    wp_redirect(wp_login_url());
    exit;
  endif;
endif;

And it works, wie, yay, happy dance.