I’m thinking Young Duke Wulfram or Duke Warburton might be behind it. They’re the only Dukes who weren’t actually in Antar during the war, and I imagine the only ones powerful enough to stand a chance should the pamphlet be traced back to them. Plus, Duke Warburton is a claimant to the throne while Young Wulfram is no friend of the King and a staunch supporter of the peace movement.
I’d rule out Princess Isobel, since it clearly mentions ‘King’, unless she was just trying to test the waters and see if people were actually open to revolt in the first place.
Or maybe it was just some young, bright, idealistic printer who thought he was doing the right thing. EXECUTE THE PRESS!
Edit: The Cazarostas could also be behind it… maybe the Earl of Leoniscourt is already planning to have Katarina marry Warburton so she can convince him to stage a coup, allowing her to become the true power behind the throne?
And let’s not forget that Wulfram, the Salt Coast, and Warburton were the regions that originally refused to join Edwin.