Kathy Bates wrote:
Just after the said events, he asks Angélique for the whereabouts of Glazer the poison-maker (and beats her furiously when she plays dumb), revealing afterwards he knew she had been the Marquise of Angels, renowned thief.
Well, in this case, the main thing for Desgrez was to stop Angelique from self-harm or suicide. He did not need the informations from her.
Kathy Bates wrote:
In Anne Golon's story, he was supposed to have investigated by himself the poison makers, under no apparent charge from the Royal government and with no funding. 12-13 years before their deeds became apparent to the world at large.
Well, why not?

As we see from the history:
Sainte_Croix was arrested in the
year 1663.
https://fr.wikipedia.org/wiki/Godin_de_Sainte-Croixhttps://fr.wikipedia.org/wiki/ExiliMadame de Montespan, the mistress of King Louis XIV was accused by Voisin’s daughter and her accomplices of being a customer of La Voisin
from 1667 .....
The Marquise Brinvilliers was in the reality ARRESTED BY DESGREZ exactly
25 of March 1676 The first macabre details of the Affair of the Poisons came to light in the
fall of 1678, when Nicolas de la Reynie, the first lieutenant general of the Paris police, uncovered evidence that suggested the existence of a plot to poison the king.
During his three-year investigation, La Reynie encountered a loosely knit community of magicians, sorceresses, and renegade priests who formed the nucleus of the criminal magical underworld of Paris.
I think, it is more than possible that Desgrez did work for Reyne whole the years, while they would waiting for the right moment to start the Affair of the Poisons.
I think, Anne Golon did brilliant recherché

- even if sometimes she did move the date a bit forward
or backward .....
Did you know the affair with the little boy selling waffles and the kings brother did really happen? .
I did lot of recherché, and did put it in the Forum in Czech language. I am happy to be abel to discus about it as well here on the english forum.
I wish my French (and English as well) would be much better.