Merrugio
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.