Thanks a lot for your answer (and for all your work !),
I think I forgot an important piece of information :
When I wrote:
erase manual grades received by a student for the questions they createdI was of course speaking of the (local) manual grade in the plugin's table (qcreate_grades) and not the central grade.
And also I was considering the case of a student request to erase his information because I think that if this is a teacher request no record should be deleted in qcreate_grades.
But if you think that for a student request I should also not delete any record, I will modify my code (easy to do !)
OK, for a teacher request I will just replace the teacher's id with 0 and preserve the comment.
Again thank a lot for your help.