woocommerce_add_order_item_meta will allow you to hook into the order creation process, its fired right after an error is checked on an item, so you know the item is legit. You could implement this with your get_post_meta function.
As for the the meta display, there are a couple things going on. The email and the order details page are both displaying an order, so when it comes to their order display they both use the WC_Order_Item_Meta class display function, in this function is an action called woocommerce_order_items_meta_display which you can use to display your order item meta (use some variation of echo). The cart, on the other hand, is showing a cart item. Woocommerce treats it differently than the former, in this case you’ll need the WC_Cart function called get_item_data where you will see a filter called woocommerce_get_item_data. This filter allows you to manipulate the item data before it is passed to the view for display.
Take a look at those links, you’ll possibly need to reference the action or filter arguments.