It is very important to keep your sweet potato cubes evenly sized, they will cook more evenly and reduce the risk of burnt pieces.
If the coconut oil is still solid, simply add to the baking sheet and toss in the oven for 2-3 mins, to melt the oil.
If you feel the roasting sweet potatoes are getting too dark, but are still firm to the touch, simply turn the oven off and let them sit in the oven for another 5-10 minutes.