I've solved the syntax problem with the LastOrderDate and have that working and miraculously that also eliminated the Dreamweaver error message. New update statement:
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$ExecuteQuery->Statement = "UPDATE customer_orders SET customer_orders.LastOrderDate = '".$OnlineOrders->getColumnVal("OrderDate") ."',customer_orders.OrderTotals = customer_orders.OrderTotals + " . $OnlineOrders->getColumnVal('OrderSubTotal') ." WHERE customer_orders.Email = '". $OnlineOrders->getColumnVal('OrderEmail') ."'";