I respect both sides of this argument.
If I am the only one in the city practicing safe water protection, what is the point. This is a system fail.
For good information and pictures of why backflow devices fail.
www.backflowinformation.com
I appreciate that you already have done substantial research.
The frustrating part for me is that in one region irrigation backflow can be a low hazard device (double check below grade).
In other regions it must be a high hazard device (PVB or RP).
I am happy to be involved in the water quality field. I began my adventure as a Sprinkler Contractor.