Outer sills sort of are structural really, because the chassis is so weedy in the middle. The strength in the middle of the van is made up of the strength if the whole structure, floor, pillars, sills, roof, chassis etc. but i take your point paradox, a hole in the outer sill shouldn't fail the mot.
My MOT blokey once said technically he can pass it if it doesn't have any sills, outriggers or jacking points at all, apart from the front n/side one (it's within 30cm of the brake compensator). I said don't be silly and he showed me a diagram with the bits considered to be the chassis/structural shaded in. We have these discussions quite often. He added the caveat that of course people don't drive vans around without these parts so it's a technicality and really you should fix them, but he can't fail a van for corrosion in these parts.
