The reason we don't see Ace burn down the house is because it's a story about change, but particularly about Ace changing and growing up - nearly the whole season is about different aspects of Ace growing up from the teenager she is when she meets the Doctor to the adult she is at the end. (Three of the four S26 episodes revolve around Ace's past and are basically the Doctor throwing her into various rites of passage.) The point of Ghost Light, from a character development perspective, is that Ace isn't that scared, angry kid any more, she's becoming an adult who understands herself and the world better than she did.