Preheat oven to 450°.  Prick sweet potatoes all over with a fork and roast on a small parchment-lined rimmed baking sheet until tender, about 30 to 60 minutes, depending on size.   Let sit until cool enough to handle.

Place chicken in a large bowl and season with salt.  Add garlic, 2 tablespoons lemon juice, and 2 tablespoons olive oil.  Toss to combine. Let sit at room temperature at least 30 minutes or cover and chill up to 12 hours.  Remove chicken from marinade, draining off any excess; discard marinade.  Set chicken aside.

Toss chickpeas, olives & feta with remaining 2 tablespoons olive oil and remaining 1 tablespoon lemon juice in a large bowl; season chickpea salad with salt.  Set aside.

Heat 1 tablespoon olive oil in a large ovenproof skillet, preferably cast iron, over medium-high.  Cook chicken, skin side down, until skin is very brown (it should get very dark; as long as you don’t smell it outright burning it will be all the better with some char), about 5 minutes.  Transfer the skillet to the oven and roast, keeping skin side down, until cooked through, 18–22 minutes. The internal temperature at the thickest part of the thigh should be at least 165°.  

About 1 minute before removing chicken from oven, toss rosemary sprigs into skillet.  Remove from the oven.  Place chicken, skin side up, on a plate along with rosemary sprigs.  Set skillet over medium-high.  Cook orange slices just until golden and slightly softened, about 30 seconds per side.  Transfer to plate with chicken.

Tear open sweet potatoes and arrange big sections on a large platter.  Place chicken, along with any accumulated juices, around the sweet potatoes, then top with orange slices, rosemary leaves and some of the chickpea salad.  Squeeze orange wedges over everything when at the table.  Offer additional chickpea salad when serving.