The more realistic question is "Just when does Diplomatic Immunity stop ?".
When I studied for my Law degree I remember quite distinctly there was the case of the "Sales Representative" who visited a customer company and who seduced a company employee there. He was subsequently prosecuted.
He pleaded not guilty saying he only went to the company as a sales representative on behalf of his employers.
The judge asked him: "What if you had killed her ? Would you then have expected me to hang your company by its company seal ?".
Is this not a simple example of how to decide where and when "Diplomatic Immunity" starts and finishes ? The lawyers behind DSK perhaps want to avoid a long drawn out legal battle, but they must have other convincing reasons to frustrate the plaintiff, Nafissatou Diallo.