The answer to my particular problem: "How to get an existing Unity ParticleSystem to play through script" (slightly more descriptive title).
Has been solved. Please note further research is required to understand why the answer worked.
One setting in the ParticleSystem is called Prewarm.
Enabling that setting allowed the system.Play and system.Stop code to start and stop the particlesystem. As previously stated, I as yet do not know why changing that setting to true (enabled) allowed the code to work.
No proper research was done.
Unite Berlin 2018 - What's new in the Unity Particle System
I was simply playing with settings, one at a time.
answered Mar 7 '19 at 18:11