It seems to me that the altimeter setting being a little off in a commercial test situation has more to do with professionalism than it necessarily being dangerous or not. For a CPL test in VFR conditions it obviously isn't the same issue it would be for an instrument rating but you are doing a test for a professional licence and they want to see that you are a stickler for detail because some day an oversight may come back and bite you, in a not good way. Now you claim that there was the possibility of a visual error but I find it hard to believe that an experienced pilot such as an examiner would not take account of or be unware of that and there would have to be a serious height difference between both of you for it to even manifest itself as an issue, something that I never saw in my interesting but limited experience.
Sometimes in life it's best to exercise some tact. Whether right or wrong, take it on the chin and move on. I failed my FAA single engine commercial after messing up a chandelle and a bunch of other manoeuvres after that. Up to that point I'd done literally scores of them with no problems. After the test the examiner just gave me a sympathetic smile and said that he knew I could do them but the integrity of the system relied on strict application of the rules and guidelines and that he couldn't pass me on that occasion but he looked forward to next time because he was sure I would blitz it, which I did.
Now it's a different situation of course and I didn't feel hard done by, just disappointed in myself but I was impressed by how he handled it. I actually felt good after his calm and reassuring debriefing and just maybe some examiners need a bit more training in that department but you need a tough skin to be a pilot and have to know when and when it isn't appropriate to take certain matters into the legal sphere. Honestly I wouldn't worry about the odd failed checkride, it just means that you didn't meet the standards on that occasion, for whatever reason.
Take my advice and don't pursue this any further.
Regards.