May I please ask if you have any thoughts on how a High Septon might be able to save Face in the wake of the Storm-Kings aggrandisement after the fall of House Teague? (I actually wonder if it would be possible for a High Septon to make an heir of Arlan the Avenger take a “Walk to Canossa” in expiation of his ancestors massacre of the Faithful).

Well, religious condemnation would be a good start, although it wouldn’t be called excommunication or interdict since the Faith of the Seven doesn’t have communication or confession as sacraments. 

Political conniving with regional rivals would be a start – the Arryns would no doubt be worried about being next, the Lannisters would not be happy about a new rival on their borders, and so on.

I don’t know if a “Walk to Canossa” would be possible, it would depend on internal political conditions of the Stormlands, as the IRL one wouldn’t have happened without the revolt of the German bishops and nobles.