I think you are way over thinking this. Every reference over the years has said "La Grippe" is influenza. Complications of influenza can include meningitis, or pneumonia.
I am surprised the doctor could not find reference to it, but I think it was used more colloquially rather than medically.
Is there some reason you are reluctant to accept influenza as cause of death for this relative? I suppose an obituary may shed some light, if you can find one.