The color pink can only be measured by our created metric and ocular input and not through other means of detection, it is through this that we discovered without human eyes there would be no pink. The world would look nothing like what you now see if you perceived it in any other way that how you currently do.
but the observable results that other people see will be very consistent.
Only to other humans, we have no idea what consciousness is. We can perceive as a group the life form ceases to function and it is indeed this group consensus that gives form to the fabric of existentialism and absolutism in reality. But group consensus is both easily manipulated and demonstrably incorrect in literally millions of examples. We have no perception of this event past our five senses, group confirmation nor what it would appear as to be using differing senses and mental function. Often breakthroughs in science are made by those who are wired differently and do not conform to this group consensus. We do not even have a decent handle on how certain humans perceive and experience the world around us.
Against the idea of absolutism and retaining the example of the founding fathers, they all thought nothing of owning slaves. According to moral absolutism they should have, being intelligent beings who can experience empathy, been able to figure out that slavery is wrong ans always was since after all, nothing ever changes our morals. But they did not because their morals were not informed by an absolute, their morals stem from the society and people who raised them, who in turn were taught that many immoral things were indeed moral or at least the way things were and therefore something to not rail against. Existentialism does not lead to individual notions in the macro but a socially informed change of ideas that has always happened and still is. Absolutism is disproved by the widely varying moralities any given society experience regardless of the backing social authority or religion the society operates under. The actively applied social morals of Rome 2000 years ago are nothing like the actively applied social morals of today and indeed, differing churches cannot even agree on these things absolutely. It is an understanding that not everything simply falls into black and white that lead to the abolishment of slavery (or at least an attempt) and the cessation of women being a form of property. Applied existentialism is not the same as a pure philosophical existentialism and that was my earlier point about not overly applying the later in real world circumstances. Just as Russia did not represent a pure socialist ideal and out current government (both sides) likely isn't actually representative of your thoughts and political leanings. I most certainly hold that killing is immoral but were I born an Aztek it is almost certain I would have not included necessary human sacrifice in this ideal. Just as many who think killing is wrong support it under certain circumstances.
I'm going to go smoke a pipe and read some Lovecraft.