Hi oldgrape. I should Google for the help you need, but my friend has a cockatiel, and he eats fruit, biscuits and salad bits. If you leave his cage door open, he'll come out eventually. Let him fly round (keep your doors and windows shut) and then tempt him back with something he likes eating. I really don't see any reason to cover his cage at night, providing a bright light isn't shining in on him, but I'm sure some cockatiel owners'll be able to advise you better than me. x
My cockatiels pick out the sunflower seeds only to eat, leaving peanuts etc. Have offered various fruits fresh/dried & salads but birds refuse them. They do love dandelion leaves!