Hi,
1. The data is stored in the hikashop_cart_product table before the order is created. After that the data is stored in the hikashop_order_product. So the data is available below the product name in the checkout cart, the order details page, the invoice, the email notifications...
2. There is no limit to the number of custom item fields to be displayed on a product page. We have other users of HikaShop with tens of custom item fields on the same product page. I don't see why it would be different for you.
3. That's wrong. If you use item fields, the data is stored with the purchase, not with the product. That's how the custom item field value can be displayed on the order details page, etc.
4. If nevertheless you want to use custom order fields, as Jerome wrote in his message, custom order fields can be restricted to products and/or categories since HikaShop 2.4.0. And it can even work with several products at a time.