This chapter is super interesting. There are many ways to look at it, but I think Grandfather Voll is keeping up his senile act throughout most of the chapter. Frieren seems to get it, especially when she says that he's joking later on and especially when she laughs at his joke that someone is going to kill the demon king. I think when he goes into this discussion about remembering Himmel's face and voice and how well Frieren remembers it, it hints towards that he remembers better than he lets on. Frieren challenges his facade in saying that she won't forget because it is as important as Voll's reason to defend the village. Its also interesting to see that when we see the wife, she is facing him.