In a large bowl, whisk together marmalade, scallion whites, fish sauce, soy sauce and crushed red pepper.
Season chicken lightly with salt. Rub garlic all over the pieces and under the skin and add to the bowl, turning to coat them with the marinade. Let marinate while you prepare the potatoes (up to 30 minutes at room temperature or up to 8 hours in the refrigerator).
Heat the oven to 425 degrees. If you like, you can line a sheet pan with parchment paper or foil for easy cleanup. The marmalade will probably scorch a little at the pan edges.
Put the sweet potatoes on the sheet pan and toss with 2 tablespoons olive oil and a big pinch of salt. Spread potatoes out in one layer and place in the oven. Roast for 10 minutes.
To the bowl with the chicken add remaining 2 tablespoons olive oil. Grate the zest of the lime into the bowl, then halve the lime and squeeze one half of it in as well (save the remaining half for serving). Toss well.
Add chicken skin side up to the sheet pan, nestled among the sweet potatoes (but not on top of them; make sure the chicken is touching the pan). Spoon any extra sauce over everything. Roast, tossing the sweet potatoes once or twice, until chicken is cooked through and caramelized and the sweet potatoes are soft, 25 to 35 minutes longer. Taste and add more salt and lime juice if needed.