You didn't even address the most critical question: is the light exactly the same?
ISO 100 and ISO 200 are NOT the same.
I am not a videographer - in fact I know essentially nothing about it - but I can guess that the reason that ISO changes automatically is that it can be done completely silently by the camera. Changing shutter or aperture means a physical change, and one that can make itself apparent on the resulting video, either by literally hearing the controls or by causing camera shake.
_____ Brian... a bicoastal Nikonian and Team Member
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