The HoH:TL states that the Redcap delivering the declaration of the Wizard's War is the legal witness of the declaration, but that some tribunals might have more elaborate procedures - so a quaesitorial witness might be a likely local custom. The role of the Redcap also gives them their usual possible role of being able to tweak things by "delaying" the message a bit in order to buy time for one or more of the parties involved.
I still wouldn't advocate giving the Quaesitors the power to regulate or deny Wizard's War. They might - granted you have a tribunal that have a local custom of the quaesitor giving witness to the declaration - haul the process but not entirely stop it. Otherwise the Quaesitors only power is to ensure that the proper procedures are followed.
As another testiment to the fact that declaring Wizard's War is a legal right no matter the motives or reasons, and thus cannot be denied by the Quaesitors, can be found i HoH:TL (p. 57) under the headline "Lawful Tyranny". This paragraph is about the trouble with magi under charge of a crime declaring Wizard's War against the "prosecuting principle" - the magus who is going to lead the case against them - and thus undermining hermetic law! Here, again, the problem with Wizard's Wars is not solved by law or quaesitorial control but by ensuring that such an approach would be very unattractive by either transferring the case to a more powerful magus that might not be bested in a War, preparing to hide the targeted prosecuting principle, or by using Guernici advocates defended by Hoplites.
Just remember that while two warring magi have forfeit immunity it is only against each other - thus third parties can only legally become directly involved if they themselves declare a Wizard's War or if a part in the War does something to forfeit their immunity versus said third party.
I totally agree. And at the same time a legal mechanism, namely the possibility of Wizard's War, serves to balance certain other interests, namely more warlike magi that might otherwise leave the Order, undermine or even war it (Flambeau...).
YMMV and even though this is how the 5th edition HoH:TL present hermetic law it also emphazised that this is traditional hermetic law and that there's plenty of room for changing and adapting this to one's own saga. These differences, and that the published line also supports it explicitly, is IMO one of the strengths of Ars Magica. I prefer to run a traditionalist approach to hermetic law and thus Wizard's War is an "inalienable" right, and the tab on its use is the powerbalance within the Order stressing that in spite of Trianoma and Bonisagus (the Parma) efforts the Order is still a bit frail. Or maybe that's exactly why their efforts are so worth celebrating. I prefer to run a traditionalist hermetic law - even if I don't personally find it endearing - but not as a fixed absolute, especially since I have plans of plots contesting it and am also planning a character who's a transitionalist Guernici advocate - the blind Jew from Prague mentioned in an earlier thread