It is a mild abrasive and polishes the plastic very well, as well as removing small scratches and he said it should remove the discoloration.
Putting aside a "stain" from smoke or something similar, the yellowing of plastics is a sign of a molecular level break down of the polymer chain, in short oxidation...
Simply and straight forward put, there is no way to polish it off or remove it without physically removing that (layer) area, just like removing rust on metal, and just like rust on metal once removed unless you 100% seal the surface it will quickly come back... And just like rusty metal the actual physical properties of the plastic are failing as well, that is why you are seeing the small cracks...
Like I said above it's on the molecular level, the plastic is falling apart, and there is really nothing that can be done to restore it...
What caused it and why yours seems to be excelerated is only speculation, but UV light and excess heat are known to speed up the process, as well as some chemicals (possibly something used to clean it, by you or even AA...)
I do hope in the end that AA at least replaces the helmet for you, and this has a happy ending...