will seems to accept his granny's death even though he missses her.however,what causes him to question the death itself.what exactly is he questioning exactly and who does he ask for answers?

I don't see will questioning Mattie's death rather than life and death itself. He thinks it ironic that Mattie used to tell stories about interesting deaths and now she is dead in a rather bizarre way. I think there is exploration of a child regarding the concept of death.