Hello julielai ! I like your thought.
The only difference is whether he is aware of his own theory or not, maybe!

Thank you, Mister M! I had only found costly facsimiles.

I'll also have a root round some London s/h bookshops later this week. It would be interesting to see the beast in question.

MrP

...I have a silly thought. Shouldn't descriptivists simply describe what prescriptivists do? Who are we (assuming we are all descriptivists) to prescribe how they prescribe? Doesn't that make us prescriptive descriptivists? ...

A thought experiment:

Grammarian MrA recommends rule X. It is a direct application of Latin rule X, which did not previously apply in English.

On MrA's recommendation, X displaces rule Y and becomes the norm.

Grammarian MrB discerns the misapplication, 200 years later.

Which of these responses by MrB are prescriptive, and which are descriptive?

1. X has displaced Y. X is therefore now the correct usage.

2. MrA was wrong. We must return to Y.

3. MrA was wrong. Y is equally valid.

4. X was once correct. But because of MrA's recommendation, Y is now correct.

5. MrA was guilty of an error.

6. 'Error' has no meaning in this context. 'Correct' has no meaning in this context. Y is the noise that some {Homo sapiens subsp. anglicus} make in situations P, Q, R.

7. MrA was neither wrong nor right. Any language user may instigate language change.

8. Any language user may instigate language change. I may call X an error, and recommend a return to Y. If usage follows my recommendation, Y is once more correct.

MrP

Interesting thoughts, friends.
Of course your first option is correct, MrP. However as I said earlier, prespective grammar started with the Latin influence. Nowadays, any grammarians who claims that this usage is right, and nothing more, regardless to the real usage among the speakers of a language, is referred to as a prespectivist.
The article that Julie has found is also interesting. I think the author is right, if you are a foreign language teacher, you have to tell your students about correct and wrong usages. But as long as you are flexible to the language changes, you are considered to be a descriptivist.
I haven't read anything from Crystal, except pieces from his language and linguistics encyclopedia. However, I've listened to some of his interviews with the BBC's Talking about English radio programmes. From what I've gathered through his comments, he is aware of the language change himself, he's a descriptivist. The following 2 paragraphs are taken from one of BBC's programmes:
"Phwoar! Interjections are words which express emotions and some are very old: words like "cooer", "gosh", "phew". You don't get new interjections very often, but one did arrive in the 80's. It was a sort of expression of enthusiastic desire - usually by men about women."
"Well, it's becoming very frequent, in all kinds of television programmes I've heard it used recently. Interestingly, although it was originally a male noise, it's now being used by women. Women are using it back to the men..."
Here, a prespective scholar would say that this usage by women would be inappropriate. That's the difference.
Hope that it helps,