Combine calamari, serrano pepper, Fresno pepper, lime juice, 2 tablespoons green onion, coconut milk, fish sauce, rice wine vinegar, and palm sugar together in a large bowl. Cover the bowl with plastic wrap and refrigerate until flavors blend, 2 to 3 hours. Pour calamari mixture into a colander set over a sink or bowl and drain, about 5 minutes. Discard liquid.
Heat oil in a large skillet over high heat until oil smokes. Cook and stir calamari mixture in hot oil until calamari turns white and firms, about 1 minute. Pour mixture into a large shallow serving bowl and cool to room temperature. Wrap and refrigerate until cold, about 1 hour.
Toss calamari and any accumulated juices together in the bowl. Add coconut flakes, cilantro, remaining green onion, and lime juice; stir.