Will less stress benefit my kidneys?

Probably. I can not think of any organ, illness or condition that will not benefit from less stress! Stress can result in high blood pressure which is not good for our kidneys. In chinese medicine stress is thought to have a strong negative influence on our kidneys. However, simply reducing stress is likely not enough to reverse serious kidney disease, but reducing stress is still likely to be beneficial.
No. Emotional stress is mainly related to the conscious minds negative thoughts. Kidneys function independently. My practice is in surgery and self-healing meditation to guide and help patients with severe emotional stress, cancers and chronic illnesses as a secondary support management of complementary medicine. Self healing meditation will remove your stress and help with positive thoughts.