Yes, I am equally confused but also voted for Sorcerous Sundries. I feel like if there is serious magic involved we may need serious magic to catch the killer/s and it hopefully will give us some leads from the 3 reasons to visit.

The weird thing to me is method and motivation. Why kill in such an elaborate way and in such a different way for each victim? It doesn't seem like revenge since the dragonborn was knocked out before being flayed, if it was revenge you would think that they would have tortured the dragonborn (given they seem to have access to serious magic that could incapacitate a victim). And with Selwyn Leventide why freeze him? If he had just drowned it would have been no less unpleasent a death (I would prefer to freeze than drown!) and passed off as an accident.
So that suggests they are sending a message to someone/s alive or these are ritual offerings to some evil deity?
And why kill in such varied ways? Strike to the head seems riskier than just casting a spell. Anchor presumably some sort of telekinetic spell? Cold spell then plant spell. Unless there is mind control involved and the 'murderers' are different characters each time controlled by this 'master'? I don't think anyone other than druids/rangers can cast plant controlling spells so maybe the druid was mind controlled to perform the act at insight park? Grave digger for dragonborn? Rouke for Seatower - spell or one off magic item like a scroll?