In re: redefining the word - It's not me who's doing so, if we're to believe
The Stanford Encyclopedia of Philosophy.Where I'm guilty of redefinition, if I am, it's in further distinguishing between morality and ethics than usual - once again, following in progression from (IMO) the last great thinker to seriously discuss Morality (capital intended): Nietzsche.
Also, it's not just lack of evidence that leads me to dismiss the idea of a universal morality (as a law of nature, or of the human condition); the very concept is subject to the same set of internal inconsistancies that lead many away from the idea of a monolithic G-D sort of entity. There is also not only a lack of evidence
for a universal morality, there is plenty of evidence
against such a concept.
Lack of evidence alone, as you point out, is not evidence of lack. However, lack of evidence for specific predictions we can make, integral to an idea, *is.*
What predictions can we make, if there were a universal morality? First of all, the good would flourish (selection pressure would weed out the immoral and the amoral, for not abiding by a universal Truth), and the evil would wither. Do we see this in the real world? I posit not. At best, such a force is a neutral element.
Second, if there is a universal morality, we are not equipped to understand nor define it. There have been many moral systems throughout humanity's history, plenty of which have/do claime(d) to be an expression of 'The' morality - generally at odds with other systems claiming the exact same thing. Basically, if there is a universal morality that affects all conceivable conscientious beings, then humans are either i)not moral beings; ii)not conscientious beings; or iii)living contrarily to such a natural force (i.e. 'super'natural beings).
Third, and related to the first point, you've yet to address the need for enforcement of a moral law, for it to have any sort of meaningful impact on the world, or the creatures living thereon. This role has universally been filled by a human being, or a set of humans, and applied to the actions of humans. It seems, at best, unlikely that a universal force would only manifest in one way - and one congruent with a world in which it doesn't exist, at that.
Your second point is valid - in a world without a universal morality. It puts morality in the same position as the concept of a vaccuum, or the number zero, or the concept of 'Pure Reason.' A useful tool; a symbol - but not something with an existence value of 1. Unless you're arguing from a strict Platonic position, one where Ideals somehow have more existance than the [ostensibly flawed] representations of them we see in the real world.
Your critique of the points I've raised thus far are precise and valid; however, it's not the whole of the story. Perhaps I'm wrong on these, as well - I look forward to your reply.
edit: formatting error.