It's very easy to hindsight and say there are a lot of ways to arrive at the same conclusion. If we accept your statements as:
1. The concept of natural rights is the most efficient and best morality
2. There are many ways of arriving to this conclusion (ie. no religious component required)
We would expect to see people independently deriving these conclusions in isolation. But we simply haven't. In fact, we see very different basic foundations of systems of morality other than life, liberty and property. One (or both) of your premises must be flawed.
Whether the Enlightenment thinkers were accepted by religious authorities of their days is irrelevant. I'm not making the case that they were mainline religious thinkers (they were progressive, some radically so), but that their philosophy was rooted in assumptions which we today would recognize as religious.
As for words being meaningless, yes, I actually do think your definition is pretty close to the mark. And your argument in favor of the meaning of language is basically a rejection of twentieth century analytic philosophy. In that regard, I think you're a little outclassed to say its "poorly thought". I know I'm a broken record on here, but read
Quine. (Or the
wiki on it).
I'm not making an argument for objective morality at all. I'm not defending the philosophy of the enlightenment, either. I'm also not necessarily attacking the structure of government devised in the constitution.
Modern progressive thought is that we've tossed off the shackles of the foolish past, and moved forward into "scientific" morality, government, based on
logic and
rationality. All they've done is replaced one religion with another, until modernists actually completely rid themselves of all notion of morality in terms of right and wrong and fully embrace complete pragmatism (which you are arguing for).
I'm actually just fine with this, probably moreso than you actually are. I think a society's morality should be judged solely by it's ability to prolong that society's existence. In this regard, I am extremely pro-evolution, pro-speciation, and I see that with finite resources the society that adapts best will win. This is the only "scientific" test.
However, as I pointed out above, this path is fraught with dangers. It puts your morality in the crucible of time and makes it very vulnerable to being...wrong. In my regime, the victory of the Axis in WWII would prove that their morality was superior to ours (or their generals - in the end, pragmatism sees no difference, because their morality produced their generals). It also showed that the morality of the north was superior to that of the south (in spite of their generals). And with competing societies with no clear advantage, it demonstrates that neither morality is superior.
This means there is no right and wrong, only effective and ineffective. Is it right or wrong for a combustion reaction to produce water and carbon dioxide? Is it right or wrong for orbits to be elliptical? Is it right or wrong for irrational numbers to exist? These are meaningless questions to science - although the latter were not, to religion. In the absence of objective morality, it is the same level of absurdity to ask whether a society should have slaves, or commit genocide, on the basis of right and wrong. Are you comfortable with this?