Combine the rice, 2 1/2 cups water and a big pinch of salt in a medium saucepan; bring to a simmer over medium-high heat. Stir, then cover, reduce the heat to low and cook until the liquid is absorbed and the rice is tender, about 15 minutes. Add 1 1/2 teaspoons coconut oil, 1 tablespoon vinegar and the scallion whites to the rice and stir; set aside to cool slightly.
Meanwhile, combine the chile pepper, pineapple juice and lime juice in a large nonstick skillet over medium-high heat and bring to a simmer. Reduce the heat to medium and cook until reduced to a thin glaze, about 10 minutes. Transfer the sauce to a small bowl and wipe out the skillet. Toss the cabbage, the remaining 1 teaspoon vinegar and a pinch of salt in another small bowl; set aside.
Season the tuna with salt on both sides. Heat the remaining 2 tablespoons coconut oil in the reserved skillet over high heat until almost smoking. Add the tuna and cook until well browned, 1 to 2 minutes per side. Transfer to a cutting board.
Divide the rice among shallow bowls. Slice the tuna and lay over the rice along with the cabbage and avocado; drizzle the sauce on top. Sprinkle with the scallion greens and corn nuts.