Is it obligatory for a widow to discard all the clothes she used to wear before the expiration of her iddah, and is it impermissible for her to wear them after the iddah ends?

It is neither obligatory nor recommended for a woman to dispose of the clothes she wore during her 'iddah (waiting period) after her husband's death. Believing this to be necessary is an innovation (bid'ah). The Prophet, peace and blessings be upon him, said: "Beware of newly invented matters, for every innovation is misguidance."