Preheat oven to 375°F. Thinly slice one-third of onion. Toss slices with 1/4 cup orange juice, 1/4 cup vinegar, and 1/2 tsp. salt in a small bowl; set aside. Pulse chickpeas, garlic, coriander, cumin, 3/4 cup parsley, 1 tsp. salt, and remanining onion in a food processor until very coarsely chopped, about 30 seconds. Add flour and baking powder and pulse a few more times to combine. Lightly oil a rimmed baking sheet. Using a 1/4-cup measuring cup, portion out 12 ping pong-sized balls and place on sheet, spacing 1″ apart. Gently press down on tops to flatten slightly. Brush tops with 1 Tbsp. oil. Bake falafel, turning balls halfway through, until golden brown and crispy, 2025 minutes. Wrap pitas in foil and warm in oven during last 5 minutes of baking. Meanwhile, whisk 1/4 cup oil and remaining 1/4 cup plus 1 tablespoon orange juice, 2 tsp. vinegar, and 1 tsp. salt in a medium bowl. Whisk in tahini and 1/4 cup water until a smooth sauce forms. Whisk 2 Tbsp. pickling liquid from reserved onions with remaining 3 Tbsp. oil in another medium bowl. Add cabbage, tomatoes, and onions and toss to coat. Cut each pita into 4 pieces. Drizzle falafel with 2 Tbsp. tahini sauce and top with remaining 1/4 cup parsley. Serve with pita, hummus, salad, and remaining tahini sauce alongside. Falafel can be made 5 days ahead; store in an airtight container and chill, or freeze up to 1 month.