There are several things here:

1. As others have argued, there are other positive consequences of going vegan besides the direct impact on the change in the demand. These include
   1. demonstrating that a vegan diet is healthy and sustainable in the long-term and can be achieved by people such as the individual in question
   2. provide data for long term studies on vegan health,  nutrition and longevity
   3. increase demand for vegan products including vegan options at restaurants and grocery, meat alternatives, cultured meat, vegan alternatives in clothing, etc.
   4. the reduction in demand from animals arising from those who made dietary changes inspired by the individual
2. I disagree that eating meat is as indirect an effect on the ultimate harm (animal abuse) as contribution to GHG is. With emissions, there are a few more steps: emission --> greater concentrations in the atmosphere -- > greater radiative forcing -->  increase in temperature  --> (potentially) disruptive and destructive climatic events  --> harm individuals. That is quite different for most people than eating the flesh of an animal that was alive not too long ago and whose life was dominated by unimaginable misery and suffering.
3. When it comes to the issue of deontology, I don't think one has to embrace the philosophical position in a thoroughgoing manner to believe that one should live a life that is relatively in accordance with the principles that one believe in. And if the principle is to minimize animal suffering, then it follows that one would do one's best to consume fewer products that increase the demand for that suffering (although that in and of itself does not lead to full-fledged veganism; more on that subsequently)
4. Even in the case of emissions, while offsetting is fine, I would still be a little troubled if someone is using a private jet for frivolous reasons regardless of how much they are offsetting it. The idea of offsetting without making any effort to reduce GHG in the first place is not ideal although I am a bit of a utilitarian monster and would bite the proverbial bullet (just as my grudging approval of a meat-eater who contributes plenty of money for animal charity although I won't deny the bad taste it leaves in my mouth).
5. That said, I am fully on board - and have argued quite a bit about this myself - that one should not be expected to a 100% vegan if one is advocating on behalf of animal rights/welfare. Or more generally, the focus should not be on how close one is to some arbitrary ideal but doing what one can given all the constraints. 

Thanks for sharing the details of this research -  it is very valuable towards arriving at an accurate assessment of various interventions.  

One question with regard to the methodology of these RCTs is when and for how long did they record the consumption pattern of the participants following the intervention? Specifically, do we have any insights on short-term vs long-term impact of such interventions focused on behavioral change? 

Also,  I understand that you report the results as SMD. However, it is quite likely that there is a small minority in the treatment group in these interventions that probably contribute to most of the difference that is observed. Do we know anything about the percentage of individuals who are likely to make considerable changes to their dietary patterns based on these interventions? 

Thanks for the comment. It is true the positive ENEs are part of the Tarsney model but there is no value assigned to the counterfactual scenario there (implicitly the value is 0). In fact, ENEs are relevant to the model insofar as they represent events that that nullify the extinction risk mitigation effort. There is no consideration of the future possibility of such scenarios and how that might diverge from the one where humanity's existence continues.

It is quite possible that human survival "improves the odds" of better outcomes as you say, but I am curious if there has been a more comprehensive exploration of this question.  Has there been an analysis examining the likelihood of post-extinction life forms and consideration of the various evolution scenarios? In the absence of that, this seems rather hand-wavy claim and while that is not in and of itself a reason to reject something, the case for longtermism needs either (a) a less rigorous argument that the overall probability distribution for intervention is favorable and not just expected value or (b) a fairly robust argument that at least the expected value is higher. 

I've heard this point being made elsewhere too but I am not sure I fully understand that. What exactly are the values  on reflection you are referring to here? Is it values that is typically shared by those with a utilitarian bent or other philosophical schools that focus roughly on the well-being of all beings that are capable of experiencing pleasure and pain. A value system that is not narrowly focused on maximization for a minority at the exclusion of others?  

Now even in the real world the systems are setup in clear violation of such principles which is part of the reason for inequality, exploitation, marginalization etc.  And while one may argue that over centuries we would become more enlightened to collectively recognize these evils, it is not entirely obvious we would eliminate them. 

In any event, why do we assume that a different advanced civilization (especially one arising post-extinction from some of our common ancestors) would not converge to something like it especially since we recognize that our source of empathy and cooperation that form the basis for more sophisticated altruistic goals have played a role in our survival as a species? 

Maybe I am missing something but even probabilistically  speaking why assume one is more likely than the other?  

Interesting considerations and if one accepts that these developments will happen within about 500 years from now, then that sets the upper bound for when the entire extinction risk events will occur in the future? 

Is there any empirical evidence to back up the claim that following the conventional definition of veganism leads to greater overall harm reduction rather than thinking in more consequential terms ? Also, unless I am mistaken, the utilitarian argument for rule-of-thumb applies in a context where we are either faced with an inability to determine the right course of action (owing to uncertainties in estimates of potential outcomes, say) or when the decision that emerges from such a calculation runs strongly counter to common sense.

I don’t believe either is the case with the definition of veganism. It is not common-sensical to avoid products with trace elements of animal ingredients for example.

Thanks for the comment. I suspect there are a couple of distinct elements that have been conflated in your arguments that I will try to disentangle. 

As far as practical considerations in the context of personal changes to limit harm towards animals go, I not only agree with you that first-order veganism is sensible, it is also one of the key reasons why I am a 99% first-order vegan. Forget animals, I am just being kind to myself and eliminating decision fatigue by following a simple rule that says : animal products, no go.  It just makes things so much more convenient and I would certainly recommend that to others too.

However, practical strategies, mental tricks and hacks should not be mistaken for ethical principles. I am sure you will agree that the  latter requires reasoning and justification not subjected to the whims of mental hacks.  If the community reifies those practical steps as a core component of the ethical baseline to be considered an adequate supporter/defender of animal welfare,  then it is clearly drifting away from the primary considerations that brought it together in the first place.

Thanks for the response. You've summarised the post very well except that, more than limiting intellectual freedom, the convention definition leads to excessive focus on purity at the first-order at the expense of broad utilitarian considerations (think of all the vitriol that vegans throw at deserters which is so irrational). 

As for your view that without the solidarity, the veganism would not be what it is today, I am not entirely convinced. To be clear, the community of interest in this discussion is the animal advocacy one and not vegans per se (notwithstanding  the fact the two of them intersect almost completely). Here are some counter-arguments to consider:

1. Animal advocates are likely to be first-order vegans or very close to it anyway. If one voluntarily chooses to make lifestyle changes based on concern for animal suffering, then one is likely to go to significant lengths to avoid animal products.  Not everyone may go  the same distance but that's okay (or so I think).  
2. Peter Singer the philosopher who arguably has the greatest claim to influencing people on animal rights and liberation is not a strict vegan and in fact describes himself as being "flexible". Yuval Harari is another person who is passionate about ending industrial agriculture of animals but describes himself as "vegan-ish". If important thinkers who undoubtedly have a great influence on people refrain from using the word "vegan", then why do you  think that as a community animal advocates should not shed that label or loosen its definition?
3. Conversely, taking vegan purity to the extreme, we have people like Gary Francione who are so opposed to any welfarist progress (regardless of its consequential value) and who insist that we should avoid meat alternatives because that normalizes the idea of consuming animals. I hope we can agree that that position is counterproductive.
4. I may be extrapolating from personal experience but first-order veganism being as clearly defined (very arbitrary but very well-defined) gives adherents the sense that they are doing enough already and dilutes thinking along utilitarian lines (what if a vegan purist compares herself to someone who is 95% plant-based but convinces 3 people every month to reduce animal products by 50%).  
5. While on the one hand, vegans could be admired for being very committed to the cause, and inspire others to do the same, they may seen too distant  which could work against people making changes that they otherwise may have been open to. Again, this is speculative and in general I think it cuts both ways. 

You may be slightly mistaken about what I am stating: the ambiguity is in the official definition even if it a sensible sounding one whereas the conventional definition is well-defined ('no first-order consumption') but arbitrary. The problem arises not so much from arbitrariness in and of itself, but rather demanding strict adherence to (and unwarranted focus on) something that isn't well-justified to begin with.  That leads to all sorts of contradictions.

On the second point, I agree that the distinctions between the two examples are somewhat arbitrary. One may argue that perhaps animal-testing in many instances is unnecessary (turns out several are based on methods and assumptions that have been around for a century and have persisted more out of inertia despite no clear evaluation of their effectiveness) but conventional agriculture depends on pesticides but I wouldn't find that argument very convincing. 

I am NOT disputing the harm to animals from eating or consuming animal products in any-way nor do I believe that the harm itself in some sense vague or poorly defined (on the contrary, there are very few things that stand out as clearly as that).

 The distinction I am trying to draw is between first-order or direct harm from a given action and the multiple indirect - second-order and beyond - ways in which that action can lead to suffering. In the conventional definition of veganism, the focus is almost entirely on the first-order effects especially when it relates to personal identification with the term "vegan". This asymmetric focus happens at the expense of consequentialist considerations of our actions.