Hmm, if I understand it correctly, both of you are in favor of leaving the canon stand, as it concerns apprentices. OK. I would only point out that under canon, an apprentice is a magical resource, not a mundane property. It is property, albeit a precious one. Harming one is far, far more serious than harming a companion.
A reasonable point. OK.
Your point has merit. OTOH, I'm also mindful of the fact that outright mundane spying of precious secrets is still going to be heavily frowned upon, and ultimately what it matters is the importance of what is protected, not the means of spying on it. Defaulting on canon and leaving the mundane loophole is a possibility, but also it is rewording the law so that only valuable secrets are protected, not the undiscriminate private affairs of mages. That would take care of abusive uses of the law, like litigation for the accidental intellego spells you mention.
Which modern Order has relaxed considerably. The days of indiscriminate hunting non-Hermetic magicians are long past.
A rebellious apprentice skirting the Oath and going rogue is covered under the apprentices provisions and is another (rather less serious) matter entirely than willfully teaching the Parma or Hermetic magic to an exotic magic-user that has no intention of entering the Order. In the first case, the teacher is cuplable of neglicence at worst and is only expcted to cooperate in full with the hunt. In the latter case, it is an automatic March sentence, unless gretely extenuating circumstances can be proved. It's the same difference between harming one's Gift, and destroying his lab.
Quaesitors that really abuse their investigative powers (either indirectly allowed under the Peripheral Code through forfeit immunity, or directly enshrined in the Code) will have Hell to pay, both from Tribunal retribution, Wizard wars, and punishment from the Primi of the House. The Quaesitori leadership is very well aware that really blatant abuse of their police powers would quickly move the rest of the Order to strip such powers away. In the ned, all the powers of Quaesitors do rest on the goodwill of Hermetic public opinion, as expressed in Tribunals. Of late, I proposed a wording of the Q-empowering provision "I understand that should I break my Oath, this Code will not shield my crimes, and members of the Order shall inquire into my affairs and deliver their judgement. I will not presume to be inquisitor or judge of my sodales without the blessing of Tribunal, nor I will invade their mind, lest I turn justice into tyranny" Don't you think this might indeed represent a strong caveat against abuse of power, along with the enshrinement of such ? All that you may then require are some Peripheral Code rulings clarifying which are the proper investigative procedures. Granted, the more political-minded Qs skirting the edges of proper would still be commonplace, but the blatant abuses of power would be severely sanctioned.