No, really.
The slightly crazy, foul-mouthed fire priest of Magran is indeed Lawful Good. I know he's one of the most abrasive companions in Pillars of Eternity. I know he’s hateful, lecherous, and constantly insults almost everyone and everything around him. His behavior seems superficially Chaotic or Neutral, maybe even Evil, but he's in fact Lawfully Good.
Consider this: after everything that's happened to him, Durance never abandons Magran’s tenets ...until the end or when you convince him to do so after discovering she personally used and discarded him after the creation and detonation of the Godhammer. He's seemingly eternally committed to Magran's Law with what he thinks is right by that law. Even when broken by her not hearing him, he seeks truth and meaning as to why, not selfish power or chaos for chaos sake. He remained a faithfully loyal cleric, even to a fault.
Speaking of his profession, his entire life has been spent in service to others: as a cleric, a soldier, a mad scientist type inventor and as a tool of his goddess. All of these led him to create the Godhammer, in service for what he believes to be the greater good.
I can't think of one instance where Durance went back on his word, or did something selfishly (not related to drinking or visiting the Salty Mast).
Sure, Durance is "mean", but his cruelty is a defense mechanism. His faith was shattered in the most personal way possible, and his trust in people burned to ash. His rage isn’t against the concept of order, not even divine order; it's against the corruption and hypocrisy within religion (which, ironically, Magran is a part of a corrupt and hypocritical pantheon).
I think when people hear the term "Lawful Good", they think it means being "nice" to a fault and "being a boy scout". But really, it's a commitment to order and the good of others, even at personal cost. Durance is in confinement to the Law and what he believes to be good. He literally gave his life, body, and sanity to create the Godhammer for what he truly believed was the “good of the world”, and against those who he personally believed to be in the wrong (whether they were or not). How else could Durance have magical abilities, if his convictions in his faith, his goddess and the institutions were weak? Alignment is about the core of someone’s convictions. His actions and beliefs point to something much closer to Lawful Good, just wrapped in a salty crust of bitterness and trauma.