Couple of other possible clues
When I view source on the correctly working test site (chrome Browser) I see the following; but it is not on the live site (with not working image zoom).

Code:
</script><script type="text/javascript" src="min/?f=/zencart151/includes/templates/responsive_template_default/jscript/jquery/jquery-1.10.2.min.js,/zencart151/includes/templates/responsive_template_default/jscript/jquery/jquery.elevateZoom-3.0.8.min.js,/zencart151/includes/templates/responsive_template_default/jscript/jquery/jquery_product_image_zoom.js&1463221628"></script>
When I use Chrome developer tools on the live site with no zoom I see the following, so I assume jquery works ok

Code:
$(document).ready(function () {
   if(_docready)
   {
    console.log("jquery duplicate ready");
	return;
   }
  
  _docready = true;
  console.log("jquery doc ready");
  console.log("document.readyState = "+document.readyState);    
  OnPageload();