How would Jarvan react if a diplomatic envoy from another country happened to be a mage?
i will be answering this as a question of jarv.an’s reactions post the death of his father as it is well established in lore that prior to then jarv.an was an avid supporter of mage rights and actively helped those of magical original when and how he could, and so would simply treat them with kindness and respect – interestingly enough such a question of his behaviour to foreign mages has actually been detailed in lore! essentially, foreign mage envoys are required to follow the laws of stone as they were prior to the regicide of jarv.an iii with the addition of requiring an escort to ensure they follows these rules. given how that in particular is explained to a mage envoy which has admittedly been to demacia prior, we can assume the compliance escort is new but the no use of magic is original.
as for how jarv.an will react, i imagine when the envoy arrives in the great city ( something that was not detailed in the story ) jarv.an would act in a reserved but respectful manner. it is not the demacian way to remark on how other countries might be or wish to change them, but with his soul shackled his prejudices run high. this would cause him to be very careful with his wording, show very little open and true emotion around the envoy and likely only interact with them in a political sense. this will definitely give a cold and dismissive impression to many people who did not know him prior and be plainly clear how changed he is to those who did know him. i personally do head canon, based on the way the mage ( arjen ) acts, that he is one such mage envoy who would have met the prince at a younger age and i think writing a drabble on such could be interesting.