The thing to always remember with code breaches is we have to change our thinking about law and punishment. In most countries in modern day we expect due process, we expect fair trials. Good luck with that it Mythic Europe.
Any fringe case that isn't a flagrant breach of the code, it's about the politics, and also power. Piss off a powerful mage who has no political friends, there's always wizards war.
In our campaign we had a mage attack another mage. A clear breach of the code. If we took it to a tribunal, his defence would depend on the belief 3 magi who had sworn the Oath of Hermes had decided within 30 minutes of meeting this mage we would breach our oath and kill him, so he had to mount a pre-emptive defence.
We didn't attempt to prosecute this case due to politics. We are a new weak Spring covenant.
In Hibernia if the English mages press charges at tribunal they 1) are foreigners to the tribunal, and thus already unpopular, 2) took a "forceless" casting as an attack, 3) assaulted a "hedge wizard" (debatable) and definitely an ally of Hiberian magi. Hibernian tribunals have a history of indignantly putting down English trying to impose continental rules on them, so unless the PC's covenant is very unpopular (or you are playing in an alternate version of Hibernia) the English magi are likely going to wind up banished from the tribunal, owing vis to someone related to the bard, or both rather than imposing a penalty on the covenant for what the bard decided to do on his own.
What's the argument for not holding the magi responsible?
Sure, we want to believe in free will and personal independence, but a lot of that is anachronistic, and even in modern society there is an argument for culpability. Sending an agent on a mission you equip them with information and sometimes utilities that enable them to do harm that they otherwise could not. The commissioner's reputation may extend to the agent, and people are inclined to trust the agent because they trust the commissioner, and that makes them vulnerable. Harm is not exclusively a result of the agent's action but depends also on the endorsement from the commissioner, and that makes the commissioner morally responsible for anything the agent might do. They had better take care not to make sure the agent has the required skills and wisdom before dispatching them.
Now, in this particular case I reckon the bard is a lot more likely to be trusted than the magi, and that makes a lot of difference. If the bard additionally is an ally rather than an agent, it makes the whole difference.
Fair point, but even if all men are free, it is still the case that some are more free than others, and hence more culpable. True today, and a lot truer in the Middle Ages.
There is also a separation of the question of culpability and reputation- the covenant may be held not to be liable for the bards actions (which someone should point out would count as forceless casting) but might gain a reputation for irresponsible use of agents which might affect the disposition of future cases.