I disagree (with Chevon ''Claq'' Dennis). I am aware of the distinction you describe, and I agree that the two-word version should be used for quantities of something, but either version is possible for the 'any longer' meaning. (Also, I'm British, and I think of 'anymore' as being American, but I don't know whether usage statistics actually bear out that theory).
I don't think "absolutely correct" is a very useful way of looking at this, I'm afraid. I don't doubt what you say, but I've never previously been made aware of this distinction and I don't think most native speakers would recognise it, either. I definitely think "any more" should be allowed as a correct translation.