To impale means to push a sharp object through someone's body; a sodomy, even by a cat's head, is not consider impalement.
Quasimodo's back deformity is not because of a fatty deposit on his back like he tells his friend (and which would have been very easy to remove even in the Dark Ages) but the twisting and bending of the spine for genetic reasons (kyphoscoliosis).
Michel is introduced to Quasi and Duchamp and gives them each 2 kisses on the cheek, one on each side. France has a customs to use 4 kisses as a way of greeting, 2 on each cheek.