Changing the product_id is really not a good idea as all the other tables related to products use that id for reference.
If you want to sort your products easily, you should use another field. There are not much fields in the order_product table but you could use the product code. So, in your products, you could set the code of products, and then, instead of having ORDER BY product_id in that query, you could have ORDER BY order_product_code. That way, you don't even need to touch the database.
the files and images of products are all referenced in the hikashp_file table. The file_ref_id in combination with the file_type is used to determine which file is for which product and for which purpose (image or file). There is no global schema of the structure of HikaShop.