What additional information would being able to see his feet provide relevant to being "double-weighted" or not? If you prefer, choose a video of the move that shows the practitioner's feet and discuss that one.
You posted a video and asked where the guy was double-weighted. I said that, using the definition that was given by CMC --which specified 'weight evenly divided between the feet'-- then one couldn't tell from a video that didn't show the player's feet. Iow, the video did not provide enough information to answer your question about him.
Your other points address the usability of the concept, since, during a shift of bodyweight from one foot to another, there's always a point where the weight is evenly divided. Iow, if the weight is distributed *80/20" between front and back foot, then to get to "20/80", it'll be necessary to go through the "50/50" distribution. And, using that simplistic definition, one can't stand in mabu and not be double-weighted. So, either it's a Zenoic paradox, or double-weighting is not simply a matter of having equal weight distribution.
Afa as putting the concept into practice, I've said from the start that teaching someone to avoid double-weighting is like trying to explain a negative. If I told a fighter "don't let your opponent punch you in the face," it wouldn't be of much use if I couldn't tell him how to avoid it. Better yet, I'd tell him how to keep his hands up. I'd even be able to give him a method to train how to do it.
Having one's weight evenly divided is (imho) not the fault. The fault is not being able to change from that position. I.e., one idea is to get the opponent into a double-weighted situation, when you are not. For ex., in the Dalu exercise, one's "shoulder stroke" intends to hit the opponent while he is double-weighted. His response should be to shift his weight. Iow, to not be double-weighted when that shoulder stroke arrives.
However one looks at or defines it, double-weightedness implies stagnation, which is not the same as stillness.