Heat a grill or grill pan to medium-high. Rub chicken with half the oil, cumin and 1/4 teaspoon salt; brush corn with remaining oil.
Grill chicken for 5 to 7 minutes per side until the thickest parts reach 165°F. Grill corn for 8 to 10 minutes, turning frequently, until lightly charred and tender.
Rest chicken for 5 minutes, then cut into small cubes. Cut corn kernels from the cob and spread both cooked ingredients in a shallow container; cool for 10 minutes and refrigerate promptly.
Whisk yogurt, mayonnaise, jalapeños, lime juice, cilantro, scallions and remaining salt. Fold in the cooled chicken and corn and refrigerate for 30 minutes.
Toast the cut bagel faces under the broiler for 1 to 2 minutes and let cool. Put one dry lettuce leaf on each bottom and divide chicken salad evenly over the leaves.
Close the sandwiches and cut in half. Serve cold or keep refrigerated in covered containers until lunch, assembling close to serving for the best texture.