A little update here... after changing the battery with the screws from a dead N9 I further investigated with the first seller who had sent me some screws from the Lumia 930. It turned out there was a mistake on his side... When he sent me the right ones they perfectly matched the one for the N9.

Bottom line: if you can't find screws for the N9 any longer, the one for the Lumia 930 are just identical. Hope it helps.

Long live the N9!