The theory still assumes a stable speed of light, as the metric against which this expansion is occurring. Thus redshift.

With classic doppler shift, when the train moves down the tracks, it doesn't stretch the tracks. The speed of light is measuring an otherwise stable dimension of space, the "tracks," in this case.

If the premise of spacetime is the basis for this expansion of space, why doesn't the speed of light increase, in order to remain constant? Yet that would negate redshift.

So two metrics of space are being derived from the same intergalactic light. One based on the speed and the other based on the spectrum. How is it that two aspects of the same light reference entirely different kinds of space?

As Einstein said, "Space is what you measure with a ruler." The ruler being used in this theory is the speed of light. More lightyears, as it expands.

What is intergalactic light measuring, if it isn't intergalactic space?