David, you obviously posted while I was still writing my last bit, as often happens, but I don't think I've contradicted you. In fact, I completely agree with your logic.
There is one point that has not explicitly come up yet which I think is worth mentioning. In cases where the action did NOT take place, it is NOT possible to use "needn't have" to refer to it.
I will say that you are absolutely correct, with regard both to your grammatical point itself, and to the fact that it is, as you so elegantly understate it, "worth mentioning" in the context of this thread. Rather, I say that your point is essential in order to appreciate the subtle differences among the various uses of the verb "to need" and its possible synonyms in various contexts.
Need I say more?