I don't think you can conclude a relationship between PD and schizophrenia from that article. What we have is a set of substances (neurotrasmitters, factors, proteins, etc) which interact with brain cells and, depending on your particular genetic mutation, some specific group of brain cells is extremely sensitive to any unbalance in the quantity of one or more of those substances in your brain. That unbalance can also be triggered by an external environmental agent. Depending on what part of your brain is more affected you have one disease or another. But maybe something's escaping to my understanding and you are right.

With regard to stress, my personal experience makes me believe that there's a relationship between PD and stress. Don't know if it's a cause or an effect.