Indeed, I am looking for what seems to be a new kind of plugin (API) that can manipulate the grade of re-attempted quiz, and not of each question or question attempt (within a quiz attempt).
Thank you for the link. It was very helpful!
As a work around, I manipulated the question behaviour plugin. (inspired by Daniel Thies) https://moodle.org/mod/forum/discuss.php?d=328970#p1344507 to change all the question's grades as the attempt was finished. (It is a little bit confusing to students to see we take off points event when the answered correctly on a specific question, although we explain way the points are taken)
I even have a new version of the above patched plugin, that requires a core patch to the quiz form, in which I input a quiz re-attempt penalty, instead of getting it from each question. (as we have quizzes with lots of questions, and teachers asked to input the penalty only once per quiz)
If at all interesting, I was post the patch here at the forum.
Again, thank you very much Tim, as always, you are a big help! and sorry for my late response.