Hello,
1.
Regarding the "service key generated in the merchant account" the solution will just be to directly create an option through your SagePay configuration page.
2.
For the Accept and Decline url, you can also create option or use a code like this to directly form the accept/decline urls :
$notify_url = HIKASHOP_LIVE.'index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=sagepay&tmpl=component&lang='.$this->locale . $this->url_itemid;
$return_url = HIKASHOP_LIVE.'index.php?option=com_hikashop&ctrl=checkout&task=after_end&order_id='.$order->order_id . $this->url_itemid;
$cancel_url = HIKASHOP_LIVE.'index.php?option=com_hikashop&ctrl=order&task=cancel_order&order_id='.$order->order_id . $this->url_itemid;
You'll find that example on the Paypal.php file of the PayPal plugin.