I believe the consequences of eating vegan are more plausibly characterized as falling under the domain of procreation ethics, rather than that of the ethics of killing. When you eat meat, the only difference you can reasonably expect to make is affecting how many farmed animals are born in the near future, since the fate of the ones that already exist in the farms is sealed (i.e. they'll be killed no matter what) and can't be affected by our dietary choices.

So I think, rather than factory farm offsets being similar to murdering someone and then saving others, they're akin to causing someone's birth in miserable conditions (who later dies prematurely), and then 'offsetting' that harm by preventing the suffering of hundreds of other human beings.

I submit that offsetting still feels morally questionable in this scenario, but at least my intuitions are less clear here.

I think MacAskill's claim in (1) is based on a scientifically inaccurate picture of how Earth's climate and weather systems work. In my view, the harm caused by CO2 emissions clearly isn't the kind which can be undone by carbon offsetting practices.

The atmosphere is a chaotic system, meaning even small changes to it can lead to butterfly-like effects. These changes snowball into significant differences in weather patterns, affecting the timing and location of extreme weather events like cyclones, heat waves, and monsoons decades later. So, when we emit CO2 in one place and offset it elsewhere, we are almost certainly causing some people to lose their homes, be injured, fall ill or die when they otherwise wouldn't have, while making it so that a different set of people avoid those tragedies when they otherwise wouldn't have. And that is why neither carbon nor factory farm offsets undo any kind of harm. Both involve harming others and then conferring an equivalent benefit to an entirely different group of people. Thus, they can't differ in terms of acceptability, I think.

The empirical support for my claims can be found in doi: 10.1088/0951-7715/27/9/R123