Preheat the oven to 425 degrees F and place a rack on the second-to-top position. Line a rimmed baking sheet with foil leaving a 2-inch overhang on each side. Set aside.
For the tomatillo salsa: Combine the agave, tomatillos, lime juice, onions and chiles in a food processor and blend until smooth. Pour into a mixing bowl and fold in the cilantro. Season with salt and black pepper. Reserve 1/3 cup of the salsa to use for the cabbage slaw. Cover the remaining salsa with plastic wrap and refrigerate.
For the fish: Mix together the chipotle powder, cumin powder and salt in a small bowl. Sprinkle the spice rub evenly all over the fish. Let sit for 20 minutes at room temperature while you make the cabbage slaw.
For the cabbage slaw: Whisk together the reserved salsa, mayonnaise and lime zest and juice in a large mixing bowl. Season with salt and black pepper. Add the cabbages and toss to combine. Season with more salt and black pepper. Cover with plastic wrap and refrigerate.
For cooking and assembling: Heat the canola oil in a stove-top grill pan over medium-high heat until just starting to smoke. Grill the fish until charred on all sides and cooked through, 6 to 10 minutes, depending on the thickness. Transfer to a plate and flake into large pieces using a fork. Sprinkle lightly with salt.
Spread the tortilla chips onto the prepared baking sheet in a single layer, allowing some overlap. Scatter the Chihuahua cheese evenly on top and bake until the cheese is melted and the chips are warmed through, 2 to 3 minutes.
To serve, scatter the cabbage slaw all over the warm tortilla chips, using only the slaw and leaving the remaining dressing in the bowl. Top evenly with the fish, drizzle half of the tomatillo salsa all over, and garnish with the sliced radishes. Serve the remaining tomatillo salsa on the side. The nachos can be served directly on the baking sheet or you can lift the foil onto a large platter.