Did the Prophet, peace and blessings be upon him, pray for his daughter Zaynab for a lightening of the torment of the grave after her death due to her not covering herself while urinating?

What was mentioned about Zaynab is not established. What is established is that the Prophet, peace and blessings be upon him, passed by two graves whose occupants were being tormented: one of them did not cleanse himself properly from urine, and the other engaged in backbiting. It is highly probable that what was mentioned about Zaynab is a lie, because the Prophet, peace and blessings be upon him, taught his Ummah the rulings of purification and Istinja' (cleansing oneself after relieving oneself).