However, this argument doesn't apply well to the notion of uniqueness, since there is no room for movement inside unique as there is in France.

No, that's the whole point. When someone describes the Hummer as "really unique", they mean that you could make it more humdrum and it would still end up unique, because it's so far from being the same as anything else to start with. Such an object is clearly more unique than another object which, while still unique, is morphologically quite close to its nearest neighbour. Denying the existence of degrees of uniqueness is robbing the language of clearly-defined and widely-used semantics in the name of an imaginary rule.

If there were two words in English, one meaning "unique" according to the Hummer definition, and the other meaning "unique" according to the more restrictive definition, which do you think would get more use? The first? Would the second in fact get so little use as to die out? (After all, it's now not a very unique word...) OK, now I suggest that both those words were spelt "unique" and the second has already died out.

literally

My problem with this is it's my experience that most people who use it in such a fashion are unaware that they're using it metaphorically because they don't understand what the word means (even though it's quite obvious).

The problem with that, is that almost all uses of the word "literal" are metaphorical; I don't remember ever seeing it used to mean "letter-by-letter". I don't think you have to be aware that "very" and "quite" both mean "totally" in order to employ their common metaphorical uses correctly.

Peter