Roger on all. I have been a shipboard diesel mech for 20+ years, but you don't find diesel particulate filter systems, oxidation beds...on these applications (at least not yet), which is why I am looking for a standard. As for the consistency of diesel fuel, at least at the shipyard bunker level, diesel fuel is not a grade of fuel, rather it is a 'range of fuel' that sees some very large variations and yet it still carries the same label. I find it hard to believe that the 507.00 engine oil standard could be so tight you can't find it on the shelf at the average retailer, but you can tank up with just any old ULSD that comes out of the pump, which changes drastically from supplier to supplier. With my 2003 TDI there were some measurable tank to tank MPG differences, which over time I figured out what stations were providing diesel with high cetane values and high energy density values. An indirect method, but I knew what stations were selling the 50+ MPG distillate and what stations were selling the less than 45 MPG blend.
I do find a little solace in explicit comments on the Diesel Kleen bottle that states it is compatible with 2007 and later diesels equipped with a diesel particulate filter and that it won't void the OEM warranty. That tells me that they are making a conscious decision to update/control their formulas to prevent binding compounds to the emission beds/substrate/media and are trying to work in concert with the emissions systems. As for finding solace with VW, they just need to provide acceptable limits, like a MIL-PRF, that will provide the aftermarket manufacturer's with a target.
As a general rule, you can't go wrong with lowering the HFRR value of the fuel and all performance diesels are noticeably impacted by low cetane values. However, outside the combustion chamber the compounds used to lower the HFRR and raise the cetane may harm the emission systems. With a modern 'emission equipped' diesel I would never add motor oil or 2-stroke oil to my diesel fuel. However, for the aftermarket fuel additives that have not updated their labels since 2007 to formally recognize that a change occurred...they may be running the same 'pre-emission system' blends, which have the potential for causing havoc between the exhaust valve and the tail pipe.
As for the Styrofoam cup trick, I would eat them alive as my background/education is in material science, mechanical engineering, and manufacturing. The Styrofoam test is nothing more than a "solvency test." Jumping over into real engineering, it would be similar to ASTM D4752 solvent sensitivity testing. You can break down the Styrofoam cup with any short chain aromatic hydrocarbon because it is a thermoplastic (reversible reaction) not a thermoset (permanent cross-linked reaction).
If any solid guidance/bulletins hits the streets, please send it my way.