Hi,
1. It's the correct place but you can't use the same code because the context is not the same.
There, in cpanel_orders the code should look like this:
$categoryClass = hikashop_get('class.category');
$brand = $categoryClass->get($this->products[$product->product_id]->product_manufacturer_id);
echo $brand->category_name;
2. I suppose that you're talking about the setting under the "Default parameter for categories". This only applies to new modules and menu items, or menu items and modules with the "Only with products" setting set to "inherit".
So if you don't see any change to the categories listing you're looking at, it means that the module or menu item displaying that categories listing has this "Only with products" setting turned off.