Hi,
1. Yes, you need to set the same URL for all of these settings. That's because in all the cases, it's the same function of the plugin that will deal with the result, and based on the transaction information returned by PostFinance, it will confirm the order and display the thank you page, or cancel the order and redirect back to the checkout.
2. That's a good point which we should improve. The first "return URL" option is there so that you can copy/paste the URL to the merchant account URL settings. The second "return URL" option is the generic "return URL" option of all payment methods made for HikaShop, which all you to specify a URL where the customer will be redirected for the thank you page. If left empty, the system will redirect to the default thank you page of HikaShop. So no, you should not enter the value of the first "return URL" parameter in the second one or your customers will have their order cancelled after being confirmed.