After he captures Griffin’s Roost, Jon Connington says he will obfuscate the matter by writing to the Iron Throne and claiming he is merely reclaiming his lands. Is that not also an offence likely to result in brutal retaliation?

This is historically grounded. Henry Bolingbrooke of Lancaster, Richard Duke of York, Edward of York, there was a long tradition of exiles making a landing and then claiming that they were only intent on reclaiming their family lands and bygones be bygones. 

While almost always a cover for a coup, it was a good bit of political cover, because feudalism being what it was, the nobility were generally in favor of lands staying in the family and looked with deep suspicion on the monarch taking people’s lands (indeed, in the case of Henry Bolingbrooke, one of the main reasons why his coup succeeded was that the nobility really did not like Richard II seizing the Duchy of Lancaster from Henry as a matter of precedent and principle).

Thus, it made it difficult for the monarch to go all-out against the invader, in part because their vassals might be quite slow to respond to the call.