Prepare the octopus: Preheat oven to 350°F. Stir together water, sake, vinegar, soy sauce, Thai chiles, onions, garlic, ginger, tomatoes, bay leaves, and thyme sprigs in a large Dutch oven, and bring to a boil over high. Reduce heat to medium, and simmer 20 minutes. Add octopus; cover. Place Dutch oven in preheated oven, and cook until octopus is tender when pierced with a sharp knife, about 1 hour and 20 minutes.
Remove Dutch oven from oven. Uncover and cool octopus completely in cooking liquid. Remove octopus; discard liquid. Cut legs from body and head; discard body and head.
Meanwhile, prepare the Kewpie mayo sauce: Stir together Kewpie mayonnaise, sambal oelek, soy sauce, and fish sauce in a small bowl. Cover and chill until ready to serve.
Preheat grill to high (450°F to 550°F). Rub skin from octopus tentacles using a paper towel, keeping the suckers intact. Toss octopus with sesame oil, salt, and pepper, and place on oiled grates. Grill, uncovered, until well charred, 3 to 4 minutes per side. Slice into 1/2-inch pieces.
Spread about 3 teaspoons of the mayo sauce on each tortilla. Divide octopus evenly among tortillas. Sprinkle evenly with nori, daikon, pickled ginger, and microgreens.