Overhead grills -- what Americans call broilers -- are unfashionable pieces of equipment. If you're grilling indoors, a ridged pan -- which is certainly better for cooking a steak -- is the thing. But for some foods, such as fish or chicken thighs, an overhead grill is better than a pan. Fish sticks to a pan; and a thigh gives off so much fat that the pan fries it rather than grilling it. The overhead grill has the second advantage of enabling you to baste the meat. (However, I do grill boneless breasts on a pan -- see here.)
I bone the thighs: they are just as juicy filleted, and they cook faster. You lift up the exposed end of the bone and scrape away the meat with a knife until you can cut loose the other end, chopping off as many hard bits and sinews as you can. At the weekend, I marinated six boned thighs in the juice of a lime, a glug of olive oil, two garlic cloves crushed to a pulp with salt, and four whizzed dried chillis.
A half-hour soaking is adequate, I think, if the meat is going to be basted during cooking. If you left the thighs in the marinade for longer, would you be able to taste the difference? I am not convinced that you would.