Ian, you quoted Grant Haist with the statement that Phenidone A is unstable. Nobody questions that Ilford can and does make long lasting developers and a fair section of my last post here describes in excruciating detail why this is so.
When we judge shelf life of developers, we need to be aware that modern photographic paper is very immune to variations in developer compositions. When I started printing, a photo supplies store sold me (high contrast) Dektol and (low contrast) Centabrom S so I could develop for variable times in two trays for fine tuning the contrast of my prints. Long story short, nobody in my darkroom could tell the difference between the same print developed in Dektol only or Centabrom S only.
Why is this relevant here? If you have only a third of the recommended amount of Phenidone in your developer, it may act a bit slowly but will still work and yield very comparable prints. So what people describe as long shelf life in their home brew Phenidone A based developers might be the result of paper makers doing their homework very well.