Quaesitores are respected for their integrity and whatnot, but have no special rights to investigate magi.
So I am wondering if you could get a Lord Darcy situation in a Q investigation:
The Q mage only acts as witness and forensic magical backup for the lead investigator - perhaps a Redcap, or even a mundane Companion who has the deductive and people skills
On the other hand, a mundane authority could be covered under "do not interfere with the mundanes" and if they are on good enough terms with the tribunal to have a quaesitor assisting their investigation (maybe in Transylvania?)
On the other hand the special rights a Quaesitor does not have really wouldn't be held by anyone else either, you might as well reverse the roles and have the mundane with observational skills be the consultant to the Quaesitor.
The idea that any mundane authority has or even could have the right to investigate magi of the Order of Hermes is exceptionally dangerous to the Order's independence. So much so, in fact, that I would expect any magus recognizing and cooperating with such authority to be promptly charged with Endangering the Order.
This is much more likely. I have pondered the idea of a Savantism+Puissant/Affinity/Cautious With* grog on occasion for exactly this purpose, though I probably wouldn't minmax quite that hard for a character I actually intended to play.
* Options include Folk Ken(magi), Perception(clues), Hunt(tracking), Intrigue(plotting)
The Q could probably claim that the mundane investigator is an essential part of their investigative equipment and, thus, is acting on the rights of the Q when investigating. Much like, I'm sure, a Q could employ someone with the Sense (Un)Holiness power as part of determining if something is infernally tainted.
This could make for an interesting legal case, though.
I'm not sure. I think a lot of people here present the Order as being very (a) militant and (b) unified around their best interests. I don't think that's always going to be the case.
Interpreting the Order as acting in rigid lock-step with concern to the law is, imo, a personal take on how to present the setting. As many Magi are going to the lack Code of Hermes ability, I feel that many wizards are going to be certain of their rights only in the most black-and-white situations. Don't talk to demons, you can kill people who break into your sanctum, no scrying on people, etc.
There are definitely going to be cases where a Magi goes along with a investigation or trial from a mundane authority because the alternative is dangerous, complicated, or makes their lives more difficult. The fact that this sets a precedent on the autonomy of the order is of far less concern to them. Solving it by placating the authority with some mixture of compliance and bribes may be needed, especially if the covenant doesn't completely own the land it sits on.
Oh, I don't think they'd be convicted- I think it would be rather a messy tribunal case that no on agreed on and probably end up with a slap-on-the-wrist vis fine unless one side or that other is particularly politically unpopular. But I would be overwhelmingly surprised if there wasn't at least one influential, conservative senior magus who was sufficiently jealous of his and the Order's prerogatives to bring charges in the first place, so yeah, in most Tribunals I would expect charges.
This is pretty similar in concept to how Theban Qs operate - the people who are leading the investigation are still magi, but the idea that Qs can act in a more advisory role is already present. From The Sundered Eagle:
In the Tribunal of Thebes the polity (in the form of a board of magistrates, see later) conducts investigations into potential breaches of the Code of Hermes, rather than the Quaesitores. Furthermore, another faculty traditionally devolved to House Guernicus — the review, updating, and expansion of the Peripheral Code — is also performed by the polity. Rather than emasculating the House, Theban magi (Guernici included) argue that this permits them to be more than simply investigators and judges, and removes an onerous and sometimes time-consuming duty from their backs. Thanks to their training, members of House Guernicus remain the experts in Hermetic Law, and are often used as expert witnesses or sources of information, which enhances rather than diminishes their prestige in the Order.
To have a Lord Darcy situation where a mundane noble is leading the investigation you would need to have a tribunal which held that breaking mundane laws is a violation of non-interferance with mundanes, and the tribunal was willing to assign a quaesitor to helping mundanes investigate crimes by mages.
If the Quaestori was perforing an investigation against you, and employing a Companion to do so, I am pretty sure killing said Companion would not help your case. Even if the lead was a Companion.
(Having said that, the House is likely sensitive to the weaknesses in the personalities of the Magi so would only employ one would it serve the needs to Justice.)