Photography: Camera sensor size difference

Makes sense. I presume the multicoating's party trick is to tame glare/reflections ?

Well, I need something to kill excessive light (I want to keep the aperture wide and ISO low), an ND would be the obvious choice (or a set) - but I want it to be infinitely variable (don't ask why!) - So I was thinking of stacking two polarizers a circular and a linear and turning the front filter to adjust the light getting to the camera. (?)

the lens will probably vignette. and you'll have all sorts of problems if you pan the shot - as the light angle changes, so will your available light.

why do you want infinitely variable ? and why via filters?

I also want something to stick on the front of my Nikon 80-200 f2.8 because it cost a small fortune and I am fucked if I scratch or otherwise knacker the front element. What do you reckon here, a UV or just a plain bit of glass (whatever they are called?).

no! far more likely to damage the front of a lens this way. leave the UV off, keep a hood on all the time