> This is about what we can measure, not what we already have.
That is not so, this discussion started with a claim that we already have observational proof that distinguishes between the two scenarios. I have already suggested that the Hubble constant could, in principle, be measured without cosmological assumptions, but we do not, AFAIK, have good enough data, and you have not offered anything to alter that opinion.
>... look at how the redshift of a star changes over time instead of staying static.
Up until now, you have been very vague about what sort of measurements you have had in mind. This is getting closer, but under which scenario does the redshift stay static?
> This is that part where I see a breakdown in your logic...
All you are doing here is disagreeing with the conclusion; you have not offered any refutation of the argument that leads to it. The reason for me saying "the relevant differences must be causally dependent on what form the expansion takes" is that if they were not, then they would not be the sort of differences that could be used to choose between the alternatives. And if they were so dependent, then the mapping from what we observe from here to what would be observed there also has that dependency.
> It's very possible that we can make measurements on Earth that would tell us exactly how our telescopes would differ from a distant galaxy.
There is something very confused here, but I cannot figure out what it is. I can say that my point does not have anything to do with "how our telescopes would differ from a distant galaxy."
> Note that the proposition "We can only get this data by going to another galaxy and using telescopes there." is not part of this scenario.
Actually, Swebs' original claim was pretty much that, as it could accurately be paraphrased as "we have seen, with our telescopes, that, when observed from a different galaxy, the expansion also seems to have originated there." You seem to have put yourself into the position of defending a claim that you do not actually believe in.
Note that there are some conditionals here that you have to be careful of. What I said was that there are certain claims that, if they were made, would result in a circular argument. I did not say that Swebs had made those claims, as he had not done so; I wrote it in an attempt to forestall a trip down a dead end (so much for that!)