The fourth objection, on who the victim is, has always seemed like the strongest explanation of the deontological moral difference to me. When you offset your CO2 emissions, you haven‘t actually harmed anyone. (I’m personally inclined to place higher credence on utilitarianism than most other moral theories, so I‘m not too bothered by this, and I also think it’s certainly better than the most plausible alternative – people eat meat but don’t offset it – but regardless, interesting philosophical question.)

The Carlsmith article you linked -- post 1 of his two-post series -- seems to mostly argue against the standard arguments people might have for ethical anti-realists reasoning about ethics (i.e., he argues that neither a brute preference for consistency nor money-pumping arguments seem like the whole picture). You might be talking about the second piece in the two-post series instead?

Brian Tomasik considers more selection toward animals with faster life histories in his piece on the effects of climate change on wild animals. He seems to think it‘s not decisive (and ends up concluding that he’s basically 50–50 on the sign of the effects of climate change on overall animal suffering) for ~three reasons (paraphrasing Tomasik):

• Some of the animals with slower life histories which get replaced are often carnivorous/omnivorous, which might mean climate change increases invertebrate populations.
• Instability might also affect plants, which could lower net primary productivity and hence invertebrate populations.
• Many of the “ultimate” life forms with fast life histories will be microorganisms that we don’t put much moral weight in.

I’d be curious for how you think the arguments in the above post should change Tomasik’s view, in light of these considerations. 

I didn’t say they fell under the ethics of killing, I was using killing as an example of a generic rights violation under a plausible patient-centered deontological theory to illustrate the difference between “a rights violation happening to one person and help coming for a separate person as an offset” and “one’s harm being directly offset.”

(I agree that it seems a bit more unclear if potential people can have rights, even if they can have moral consideration, and in particular rights to not be brought into existence, but I think it’s very plausible.)

Note, however, that I think the question of whether there can be deontic side-constraints regarding our treatment of animals is unclear even conditioning on deontology. Many deontologist philosophers – like Huemer – are uncertain whether animals have “rights” (as a patient-centered deontologist would put it), even though they think (1) humans have rights and (2) animals are still deserving of moral consideration. Deontologists sometimes resort to something like “deontology for people, consequentialism for animals” (although some other deontologists, like Nozick, thought that this was insufficient for animals). 

I think offsetting emissions and offsetting meat consumption are comparable under utilitarianism, but much less comparable under most deontological moral theories, if you think animals have rights. For instance, if you killed someone and donated $5,000 to the Malaria Consortium, that seems worse – from a deontological perspective – than if you just did nothing at all, because the life you kill and the life you save are different people, and many deontological theories are built on the “separateness of persons.” In contrast, if you offset your CO2 emissions, you’re offsetting your effect on warming, so you don’t kill anyone to begin with (because it’s not like your CO2 emissions cause warming that hurts agent A, and then your offset reduces temperatures to benefit agent B). It might be similarly problematic to offset your contribution to air pollution, though, because the effects of air pollution happen near the place where the pollution actually happened. 

Why do you think excruciating pain is 10k as intense as disabling pain? If I use these conversion factors (p. 30) instead, chicken welfare campaigns seem to win. 

Do you think there are promising ways to slow down growth in aquaculture?

This post by Carl Shulman is very similar to this, I think.

Somewhat relevant (takes the hard proves-too-much stance): https://www.econlib.org/archives/2014/10/dear_identity_p.html