I am not sure if you have personally promoted very many bike races or not, but you should. You will find a person like yourself on both sides of the argument telling the promoter they are wrong on both sides. The promoter can't win, so they don't. They add additional rules to make their race fair for "most" of the racers in order to encourage the largest turnout possible. They want a full field of racers ... in some cases they want to cherry pick the fields, but that is very rare.
Their goal is to piss off the least amount possible, if you don't like their rules, don't do their races. They are likely counting on a larger number of people liking their rules and doing their event because of their special rules - likey because of people like you in the past. The same can be said for a race that only has a 45+ Masters race instead of a 40+ or 35+ race. Or a cat 3/4 race instead of a cat 3 and cat 4 or a cat 1/2/3 and cat 4. Each race will turn away a few because of how they set it up, but it is their race to run, and no rules exist which state a promoter can't have race specific rules.
So fight with them all you want, but do it from a numbers perspective if you want a change. They don't, or at least shouldn't care at all about one racer complaining. If you truely think the rule to be stupid, then get a number of racers, or better yet teams to tell the promoter the rule is bad, then the rule will change.
As for your helmet, if it is legal for all the events wear it in all the events. But if you wear it in one race and not the other, well then you are not playing by the fair rules of this race which is basically you get to pick something and then use it for the whole event to lower the cost for most racers. And specialized and giro love that you think their helmets are the best of both worlds ... marketing science at its best. I do agree that the helmets with less than 6 vents rule is getting extreme, but bring that up and get teams to back you that it is a stupid rule. Maybe it will change for next year.