A point object cannot be "slightly" in France, but it can certainly be "nearly" in France, or, if a long way from the border, "really" in France.

I'd argue that something could be slightly in France, if it were just on the other side of the border from the thing that was nearly in France. Then again, maybe that would be barely in France, and something slightly in France would have to also be partially in another country as well, as a city overlapping a border (which happens occasionally -- Kansas City, Sault Ste. Marie, and Niagara come to mind -- although I don't know any in France).

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. Unique means that there is one. If that is modified in any way, it is no longer unique. There is no wiggle room. Whereas there is a great (certainly non-zero) distance between, say, Nice and Calais or between Bordeaux and Paris.

I've got similarly little patience for people who insist that the word "literally" cannot be used metaphorically.

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).