What is the purpose of low stools? It is customary for the aveilim, or mourners, to sit on low chairs or boxes during the Shiva period. Indeed, it is possible that this practice resulted in the expression "sitting" Shiva. The intent is not for the mourners to be uncomfortable (no asceticism involved), just for them to sit lower than usual, a recognized sign of mourning.
Why are the mirrors covered? Covering the mirrors is less about law than custom. Regardless of whatever superstitious or cultural norms may have originally been behind the institution of the custom, it has become established mourning practice to cover the mirrors in the house of mourning, especially in the public rooms. This also helps remind us that Shiva is not so much about ourselves as it is a time to concentrate on the deceased.
