Breadcrumbs level limit

Well, there are a couple of ways you could achieve this. Using something like Order Bender to set a preferred category, and setting that to be the 2nd level category for each applicable post is one way of achieving this result. You could hook into the bcn_after_fill action, go through the breadcrumbs array and removing category breadcrumbs after there have been 2 (see https://mtekk.us/archives/guides/conditionally-remove-home/ for an example of removing a breadcrumb from the breadcrumb trail, you will just need more logic for figuring out what breadcrumb to remove).

The below code, in theory, probably does what you want. Note that I have not tested it so there may be a PHP warning/error message lurking in there.

add_action('bcn_after_fill', 'my_only_two');
function my_only_two($trail)
{
   if(count($trail->breadcrumbs) > 4)
   {
      $breadcrumbs = $trail->breadcrumbs;
      krsort($breadcrumbs);
      $cat_count = 0;
      foreach($breadcrumbs as $key => $breadcrumb)
      {
         if(in_array('category', $breadcrumb->get_types()))
         {
            $cat_count++;
            if($cat_count >2)
            {
               unset($trail->breadcrumbs[$key]);
            }
         }
      }
   }
}