Are there any known cases where magi or covenants has been acquitted from charges of endangering the order and/or interfering with mundanes after acting in self defence to protect their own lives and their magical resources from mundanes?
I don't know, but I always think that the Order's ruling on such matters would come down to pure pragmatism. If you did not create the situation in which mundanes threatened your resources, and your actions have not provoked a backlash, then you are fine.
So killing some worthless bandits would not be a violation, leaving one alive to run into a confessional booth and break down in tears would be, however many resources you saved in doing so.
Defense of self and one's property is entirely permitted, and is probably the most airtight defense against interference against the Mundanes.
A magus will probably have to explain his reasoning and the justice to the amount of force used in self-defense, but no tribunal is going to pass a judgement that a magi should DIE or give up his stuff to avoid trouble with the mundanes.
That depends on the fall-out of that self-defense. Your sodales will judge you according to the hassle you caused them by it, and may well become sticklers about "Just how did you end up in that mess anyway?".
It also depends on there being witnesses who could identify the magus as a member of the Order (aka. the Flambeau Defence - if there are no witnesses, there was no crime!)
I know how to argue in defence of it, I was just wondering if there was any established case law on the matter in print.
The right to self-defense is a principal requirement of the Code. It doesn't need a case. Everyone has the right to defend themselves against an illegal attack (including serfs against nobles, not that a court will necessarily be friendly to the argument).
However, you may be looking for something beyond a precedent.
We may be about to explore this in our saga. A maga we are acquainted with seems to have killed a noble's son in defense of her honor - that is to say, it seems that she was defending herself from unwanted attention, and it's not yet clear what that attention was, annoyance, assault, or rape. This plot seems to aimed at our resident quaesitor; there are likely to yet be twists.
As someone else on this board has pointed out, it is all about which Magi are sitting on the Tribunal that is hearing the case. And that can depend on the Tribunal as well.
Good point, a distinction is to be drawn between the "objective" law and the subjective politics.
Wizard's Grimoire from ArM4 (and maybe in the original ArM3 version). Look up the cases concerning one Trentus of Flambeau located in Iberia.
Now witnesses. Check!
Also found a couple interesting ones concerning enemies of the order, amongst them one where the Duke of Brittany was declared an enemy of the order du to his prosecution of magi, but where a tribunal of quaesitores ruled that one could still be called to account for interfering with mundanes if taking action against him.
Seems like even when fighting enemies of the order one has to take great care.
Or do not make it look like Magic. Disease or accidents can happen.