In a large bowl, combine the yogurt, cilantro and scallions; season with salt and pepper. Measure out 1 cup and transfer to a small serving bowl; refrigerate. Measure out 1/4 cup of the mixture, transfer to a medium bowl and set aside. Season the chicken with salt and pepper; add to the large bowl with the remaining yogurt mixture and toss to coat. Let marinate for about 30 minutes.
Meanwhile, stir 2 tablespoons chutney, 1 tablespoon EVOO and the mustard into the medium bowl of yogurt mixture.
Preheat a large grill pan over medium heat. Toss the sweet potatoes on a plate with the remaining 2 tablespoons EVOO; season with salt and pepper. Arrange on the grill pan and cook, turning once, until lightly charred, 10 to 15 minutes; transfer to a plate. Let cool before cutting into bite-size pieces; toss with the yogurt chutney.
Oil the grill pan and cook the chicken over medium to medium-high heat, turning once, until cooked through, 10 to 12 minutes. Serve with the remaining 4 tablespoons chutney, the refrigerated yogurt sauce and the sweet potato salad, topped with the cilantro leaves.