If the Count attacks us, it is treason. We are a servant of the king, an unjustified attack on a kings vassal is an attack on the king himself. If we go to an authority, either the count, his marshal or the duke, we are doing things legitimately, as long as we do things legitimately we have the legal right to demand justice, any refusal to allow us to do so is very bad form, outright illegal if he handles that wrong.
I do not think we should kidnap the merchant, we should charge him, doing so forces our enemies into a situation they can't win in.
Also, the merchant may very well give up the count in exchange for leniency.
This merchant tried to *kill* us, and now is sending armed men into our lands, i would be surprised if he could worm himself out of that, counts son in law or no.
If the count attacks us succesfully, "finds" some documents somewhere that indicate rebellious things, and wins before anyone can intervene, nothing happens. The Duke won't risk a civil war over one lowly knight.
Remember that the links are pretty weak. We got some peasants saying that he did so, nothing else. Now, give a peasant a bag full of money ,and he says what you want. The merchant just has to deny all the claims, no more, no less.
The wool merchant will crack because he's the front man. He knows what he is, and what being caught means, and if he doesn't we can make him.
Why would he. We got no real proof. We got 2 peasants, but can you trust those. See, for the merchant there are 2 options:
-Talk: Sentence + wrath of Count and supporters
-Don't talk: No sentence + wrath of the knight you tried to kill
Also, we can't make him talk during the trial. We don't have the right to keep him in our custody*, so we'll need to surrender the prisoner to either the Duke or the count. Now, it's kinda hard to torture people during a formal trial.
*Read, don't have the political power to afford doing that without being called a blackmailing traitor
Because, really medieval Judgings are decided by Money, political power and military force. WHo's right and who's wrong has nothing to do with it.