Magento – Order email, how to get the product Id

Here is the function where it is defined.

It gets the categoryId here:

$categoryId = (int) $this->getRequest()->getParam('id', false);

If that's not in the request path, it won't set 'current_category'.

A product can be in several categories, so when you route to the product directly, it doesn't know the category. There are several ways to find the parent categories of a product, but you will have to find the right method for you. It's easy if you don't put products in more categories but more complex if you do.

Alan Storm's answer here can get you started: https://stackoverflow.com/questions/15735324/get-a-products-parent-category-even-if-it-is-accessed-directly

   protected function _initCatagory()
        {
            Mage::dispatchEvent('catalog_controller_category_init_before', array('controller_action' => $this));
            $categoryId = (int) $this->getRequest()->getParam('id', false);
            if (!$categoryId) {
                return false;
            }

            $category = Mage::getModel('catalog/category')
                ->setStoreId(Mage::app()->getStore()->getId())
                ->load($categoryId);

            if (!Mage::helper('catalog/category')->canShow($category)) {
                return false;
            }
            Mage::getSingleton('catalog/session')->setLastVisitedCategoryId($category->getId());
            Mage::register('current_category', $category);
            try {
                Mage::dispatchEvent(
                    'catalog_controller_category_init_after',
                    array(
                        'category' => $category,
                        'controller_action' => $this
                    )
                );
            } catch (Mage_Core_Exception $e) {
                Mage::logException($e);
                return false;
            }

            return $category;
        }

When you have some configurable products created on the basis of that attribute you can't delete the attribute. But if you want to really delete that attribute, you have to delete the products first.
You will not find any hacky method for it. These attributes are saved in eav attribute table and products just get its id (attribute_id) to relate. so if you delete the attribute, you will face errors like "something is being load non-object..." etc, and you may even not be able to edit that product at backend nor it will load at frontend. so be cautious before committing such things on live.