The bridge between these foci is their interrelation, the former the ground for the latter, the latter dynamically affecting the former. The point here is that how we characterize the focus and meaning of the series informs our response to its finale.
For me, the finale worked for among the reasons Chotiner (see link below) argues. But a few points:
My take is that Hart's wife remarried. She lives in a big house. At the hospital, with what had been a family all together, she wears a wedding ring. This is all to me the husk of family, shaped by irreducible but permanently fractured bonds, with the ineffable sadness of irretrievable loss conveyed in Hart's tears.
In Cohle's descent into and ascent from the heart of darkness, from his experience of aborning death, he confronts the very limits of his brittle, soul destroying and defensive nihilistic cynicism, parading as a superior apprehension of the nature of things. That argument from both futility and his own sense of superiority was always in tension with his inexorable drive, his obsession, to do right and discharge his self-perceived debt, as evident in needing compulsively to solve the series's crime. It's on that moral basis, the need to set right and complete the episode 5 concocted faux resolution, that Cohle is able finally to convince Hart to join him in episode 7.
So there is for me a thematic closing of a circle as relationship finds ground for itself in crime solution, the unspeakable magnitude of the crime bracketing and defining the detectives' own imperfections, albeit haunting their lives, but giving truth and content to the idea of bad guys with their own darkness guarding against and warding off much darker and much worse than them.
The last two episodes as procedurals show the detectives in their ascending relationship fulfilling their moral lives and their competencies in being true detectives.
Final note: the disappointment over unresolved details, clues, and mysterious significances is to me beside the point. All is not well. All the unresolved aspects of the murderous cult whose tentacles spread beyond these true detectives' ken and capacity, all the tantalizing clues and false positives slung throughout the series, they are all of a piece with the vast territory of the darkness the detectives understand they cannot surmount. They must be content with one of Auden's points of light that sets off and gets a measure of human purchase on the darkness.