Mystery solved. Backstory.

It was love.
It's always love.

The Murder eliminated the fair Michelle's Husband hoping to  woo her based on their close childhood friendship. Once Michelle was a childless widow, who stood to inherit a fair sum of money, other young aristocrats came calling. So they had to die, there was no path.

Gregorio Harmit wasn't always a priest. Once he was a young aristocrat himself.  His fondest memories are of his youth boating with his friends. Especially Michell.  She was from another wealthy family, and was in his eyes his other half.  They spent many days together in their youth, as they got older they spent a few evenings conducting secret rendezvous.

However, it couldn't last. Gregorio was from a wealthy family no doubt but Michelle was from the  upper levels of aristocracy. As is the tradition her marriage was to be arranged by her family. None of it seemed to bother Michelle, she knew the  workings of society, she had enjoyed her youth and now she had to take on the mantel of society wife. It was expected, tradition,  her duty to her family, unavoidable. Gregorio on the other had was crushed. He withdrew from society, told his family he was going to join the clergy, a calling that would force him to forget. He never forgot.
Instead he obsessed.
Born into society, even as a simple acolyte he was still a man of means.
He rose quickly through the church of Boreas's ranks. He was smart, compassionate, and  his family was well connected. Through his  connections he procured hefty donations and funding, helped build a temple on top of trade winds hill overlooking the city. A spot he and Michelle used to meet.
He had statuary built, studied herbalism, learned the rituals of divination, prayed for Boreas to help him with his enchantments. The wind could see all he learned, and the  wind could animate a stone statue if the right rituals were known. Boreas was the god of  north winds and coming winter, Boreas wanted Gregorios heart cold and it become so.

He hatched a plan:
Remove the husband, and comfort the girl, rekindle the past friendship, and regain her love.

So he used his blessings to animate the largest of his  statuary, an eagle headed man, a fierce Kengu statue. He sent it to  kill the husband, leave no trace. Kill and return to you place.

Once that act was done he asked for  forgiveness from Boreas, and  received only cold wind blowing across his sanctuary. He sent condolences, attended the funeral, blessed the grounds of Michelle's manor.

Now the suitors came out of the  woodwork. Looking for a wife or good standing, a good family and due a fine inheritance. Gregorio knew none of them cared for her like he did, they had to die as well.
Again he sent his tengu statue to do dirty work, one man dropped form a height, Another stabbed.

The others killings he had to take a more personal hand in. Clairvoyance on an a manor gate. A carriage driver affected by a blindness curse at just the right moment. A horse given bull's strength to increase it's speed almost guaranteed a young aristocrat would be run down when crossing  the avenue. Gregoria was the first to rush to the scene, a priest, helping to mend the man's injuries, but of course it was too late.

Lastly there was the poison. Not Gregorio's cup of tea. He gathered the ingredients form the fields and  tucked away spaces on Tradewinds hitt top. Rare mushrooms , and  berries form vines. He spent a great deal for poisons recipe. Bought it from a sailor who said, "We use it to kill rats on the ship." Innocent enough, and Unlikely.
Making the  food  at dinner taste  "proper" was a simple act of Thaumaturgy. Watching a man die painfully over the  next few hours was  not nearly as simple.

Each time gregorio went to the funerals, each time he would stand by Michelle's side. Deliver a eulogy, give the blessings of Boreas. Offer the comforts of an old friend. Each time he would ask Boreas to forgive his deeds, pray, and open his heart letting his god know his motivation was love. Each time the  cold winds would  blow through his sanctuary telling him his god was with him.

The deaths caused a maelstrom of rumors and intrigues, some so vastly beyond the scope of Gregorios plan that he could not help but laugh to himself. The rich families of the city lashed ot at any  group with which they might have had an old grudge. Accusations flew. Then the  leaders of those families hired a rag tag group of sailors who said they could help  find the  killer.

Gregorio watched through his clairvoyance enchantment, discretely cast on Michelle's own cameo.
When that man Kip accused his Michelle of being the killer based solely on his own sexist leanings Gregorio's blood boiled! Why were none of the other men drawing dueling pistols and teaching the whelp a lesson? Were they men at all? Gregorio would not let the insult stand.

So he sent the Tengu to deal with Kip.
The Tengu failed. In the scrying bowl, he saw the Tengu fall, he saw kipps eyes fill his vision, Gregorio was seen, they would find him.
So he prayed and prepared, using the scrying pool to deduce where the investigators were searching. He knew the wretched sun priest knew nothing, he knew those of Geb could not help them, but he knew he would be found.

When they came he was prepared to battle to the death, but in the end the priest of Thodin dispelled his wards easily , and there would be no fight. Boreas' had deserted him , no breeze blew in his sanctuary. The  air was still as he proclaimed his guilt and revealed his motives. Now it's jail, stagnant air, and a trial.

Gregorio left to wonder if this was his plan all along, or the will of Boreus and the  winter winds.