Is the discard pile of runecards open and searchable for all players at any time?
If yes, I assume, you have to show the runecard you choose with "Granted Prayers" to the other players?
Rulebook says that only the cards in a Viking's hand are hidden, so yes the discard pile is open.
Yes, the card chosen for Granted Prayers should be shown to the other players. Funny, this has never come up in any of the games I've played in. Players always seem to announce which card they are taking.