basically the film isn't grainy. from what he described to me it looks suspiciously like sharpening noise. I have never known grain on colour film to look like white dots.
Airport carry-on scanners are fine. I trust my slides/negs with them and so far nothing has happened to my film, exposed or not. Now the check-in luggage scanners destroy film completely. If you leave them in your check-in luggage, you can kiss your film goodbye.
I've used NPS before at the rated speed and like most print films, it performs fine. The difference when overexposed isn't really appreciable for me. Only BnW films seem to benefit greatly from overexposure to some degree, but that's just my personal opinion of course.