I agree, if one piece (or even a small number of pieces) of evidence is in and of itself proof beyond reasonable doubt, then everything else becomes moot. But, in this instance, the relative weight of the evidence is crucial. You've provided a hypothetical example of a single piece of evidence that's overwhelming proof of culpability. But the same logic wouldn't apply to another single piece of evidence that was only marginal proof of culpability.
And, with regard to the relative weight of evidence, I'd go back again to the old "rope" analogy: Some pieces of evidence carry more weight of guilt than others. Therefore they constitute thicker "strands" of the "rope". Conversely, other pieces of evidence are less indicative of guilt, and therefore constitute thinner, weaker "strands".
If there's reasonable doubt over any one piece of evidence, then its "strand" is removed from the rope. When all the pieces of evidence are weighed up in this way (i.e. firstly for the "thickness" of the "strand", and then whether the "strand" can become part of the "rope"), there is a resultant rope - which now comprises strands of varying thickness representing pieces of evidence which remain proven beyond reasonable doubt. The jury then "pulls" on the "rope". If it holds firm, then they should vote to convict.
To apply the rope metaphor to your scenario: Any photo/video of AK stabbing Meredith would, on its own, supply a massively thick single strand to the rope. All the other strands would pale into insignificance alongside it. The jury could disregard all of the other strands of the rope - the single hugely thick strand representing the murder video/photo would ensure of its own accord that the rope would not break when tested.
But, as you say, that's purely a hypothetical in this case. There's no "smoking gun" here.