Sorry to be so late in answering your questions, Penth.
Yes, the sweet potatoes must be peeled first. You could substitute pine nuts or any other kind of nut for the walnuts, or you could use sunflower seeds or granola or wheat berries (That cereal is called Grape Nuts in the US). I wouldn't recommend using cornflakes for the reasons you mentioned. Or, you could make a crunchy topping with oatmeal, tossed with some melted butter, flour, and brown sugar and sprinkled on top of the sweet potatoes and baked until it is brown and crisp.