My student was not 'arguing'; she has an inquiring mind, and wanted to know the reasoning behind what she accepted, on my say-so, as the correct usage. And she would indeed be able 'to understand the mandative subjunctive if explained properly' - which is what led me here in the first place.

Yes, I made a muddle of it when I initially tried to explain it. However, I am quite familiar with what I had thought of as a familiar usage. I was surprised to find, here, that so many people are unfamiliar with that usage. Further, I was surprised that so many would express doubt, if not outright denial, that it could have any validity, particularly after an authoritative, though certainly not exhaustive, explanation had been provided (that is, linked to). If I was baffled myself, it was previous to receiving an explanation; after having received such, my bafflement, in that regard, ceased. Now I am baffled as to why so many others apparently remain baffled.

I hope that makes everything clear.