I paid my fine (an annoying five gold pieces) and returned to the city. Wandering around the city yielded little gossip... mostly chatter about the emperor and the dark brotherhood... but nothing about Thorimir. I found him in the city, talking with someone that a local told me was named Agramir, but their demeanor was closed and I was unable to discern anything about their relationship (beyond the very fact of a clandestine midnight meeting).
I spent another day in town, lounging about shops and again listening to local gossip... nothing new. This time, when Agramir noticed me following them he shouted, "Thief!" Before I knew it, I was in irons on my way back to the prison. Another trip, another fine.
It worked out to my advantage, though. Agramir was apparently satisfied that I would be occupied for longer than I was: neither he nor Thorimir noticed as I caught the end of their conversation: they argued over shipping and the diversity of goods available between them.
I followed Agramir hack to his house in the Talos district. I waited for him to leave, then entered his house to find evidence of grave-robbing: bones, dirt, shovels... even a manifest of what he had exhumed.
I took this evidence to Thorimir, who was agast at the thought. Eager to help, he directed me to the cemetery, where he believed Agramir would be plying his trade at that very hour.
I found Agramir in the Tirentius family mosoleum, but I had not evaded his notice: he was waiting for me, and with an armed guard. It was a harsh fight, but I prevailed, recovering proof of his deeds.
When I returned, Thorimir had donated his profits and the remainder of his inventory to the temple, and had joined the merchant society. He even gave me a ring of elemental resistance in thanks.
Jensine acknowledged and appreciated my involvement. My reward was 100 gold pieces.
Now to sell that plate armor...