What is the ruling on the prayer of one who, after finishing it, discovers ritual impurity on the garment in which he prayed, knowing that he was negligent and lazy in verifying its purity before the prayer?

The fatwa you relied upon regarding the validity of your prayer with urinary incontinence is correct, and you do not need to repeat it. You may continue to act upon it in the future, even though purity is preferable for you. According to the Malikis, there is no difference in the forgiveness granted for the impurity of incontinence, whether you continue to wear the garment and pray in it, or if you remove it and then put it on again. This is because the impurity of incontinence is forgiven for the excused person. The Malikis differed on the permissibility of a non-excused person following an excused person in prayer. However, the well-known opinion is that if the impurity is forgiven for its owner, it is also forgiven for others, so his imamate is not disliked. As for a non-excused person praying in the garment of an excused person, some scholars prohibited it.