
Introduction
I know there were more sorcerers during the first colonisation, but they were rather rare at the beginning of the second. So a specialist in metascience or alchemy could certainly help you identify the person who made the spell on the bow. You are wondering how knowing that would help us? Well, if we can tie the mage to the suspect's family, that should be enough to reopen the case. Got me?
Obviously, the file will not be removed from the archives, since too many years have passed, but the person for whom you are investigating should be satisfied.
In the Sentamau Forest, there are druids who could probably be useful to you to solve this enigma. Find where they meet and negotiate with them.

Cunclusion
Yes... the school of the Foggy Hawk was funded in part by Remio's family. This confirms my doubts... I should have done this check before I died. I may not have been a very good inspector after all. Thank you for your work. I hope that Vala will be pleased about your discoveries.
I do not have the documents your client asked you to, but I guess the information you have will answer his main question.