There's a target that is quite simply much more convenient for our purposes:
That merchant guy is clearly the traitor you seek! It makes sense: He was clearly seen consorting with the brothers at several intervals during their plan, no doubt delivering intel to them under the guise of "taking their money". Merchants tend to go everywhere there's money, and this they go everywhere in a big kingdom like this. Everyone, from smelly peasents to high level priests, patronize them after necessity, and no one really questions they do stuff beyond that they want to make money.
PLUS, he sells stuff for money! WHAT ELSE MAY HE HAVE DONE FOR MONEY?!? Clearly untrustworthy, love of money being the root of all evil and all that.
>Merchant: Find out about this, decide to leave town and become a traveling merchant, further torment the Bros.