Most non-vegans don't take vegan B12 supplements. That means this vegan product is excluded from the non-vegan's diet. The reason why non-vegans exclude it (whether they don't like it, consider it as immoral...), is not important because reasons are not health related. Whether or not someone who doesn't take the B12 supplement categorically refuses to take it, has no impact on that person's health.

I was pointing at a non-vegan bias in the way how you framed your argument: that a vegan diet is restrictive. But non-vegans also eat a restrictive diet, as they don't eat (and often refuse to eat) vegan foods. Vegans don't eat non-vegan sources of B12, and non-vegans don't eat vegan sources of B12. 

Your bias is comparable to a native English speaker who has an English bias and claims that French is a difficult language because the French people don't use those simple words like "door" and "table". So when you want to speak French, you first have to learn new words. But the fact that the French language doesn't use the words that you use, doesn't make it a difficult language. For native French people, French is an easy language.

So the crux is: a vegan diet is not difficult, but changing diet is difficult. For vegans (who learned how to eat vegan), a vegan diet is easy, just like a non-vegan diet is easy for non-vegans (who learned how to eat non-vegan).

I agree with your two sentences, but the first one is very ambiguous. You mention someone with a B12 deficiency. The way I see it, both vegans and omnivores remove sources of B12 from their diet: the vegan doesn't eat animal products that contain B12, the omnivore doesn't eat B12 supplements (or B12-enriched products that are suitable for vegans). Many omnivores even refuse to eat those vegan B12 supplements, just like vegans refuse to eat meat. Now you have someone who doesn't eat either of those B12 sources: no meat and no vegan supplement. You can call it a too restrictive, unhealthy vegan diet (because the diet doesn't contain meat), but you can equally call it a too restrictive, unhealthy omnivorous diet (because the diet doesn't contain vegan B12-sources). There is a kind of symmetry.

I'd say a healthy vegan diet is roughly as difficult as a healthy omnivorous diet, and a convenient vegan diet is roughly as easy as a convenient omnivorous diet.

Someone who is highly productive in reducing X-risks, is first highly intelligent, which means intelligent enough to know how to eat a healthy vegan diet, and second, most likely living in a wealthy environment with good access of healthy vegan food, which means able to follow the knowledge about healthy vegan diets. So that means if a person still has adverse health effects from the vegan diet, while following all knowledge about healthy vegan diets, it must be because of unknown reasons. And that seems very unlikely to me. We know so much about healthy food...

It seems that you make nothing but a very trivial claim, that if you are used to A, a change from A to B is difficult. But then you frame it like B being difficult. But it is the transition which is difficult, not B itself. As an analogy, let's discuss whether Chinese is difficult. You would say yes, because it is not your native language. It will take some effort for you to learn Chinese. But a Chinese person thinks Chinese is easy, and English is difficult. Who is right? In the end, once you have learned to speak Chinese, it is as easy as most other languages that you have learned. Some languages are objectively more difficult than others (like Dutch is probably more difficult than Afrikaans, and English is more difficult than Esperanto), and for the same reason, some diets are objectively more difficult than others. But you make it look like veganism is objectively more difficult than other diets. I disagree with that: just see how much you have to learn for a healthy diet with meat. I think a healthy vegan diet is as difficult as a healthy omnivorous diet, and an unhealthy vegan diet is as easy as an unhealthy omnivorous diet. Transitions may be difficult, but they involve one time transition costs, which become negligible in the long run. Once you have learned a foreign language, the cost of learning drops to zero, and the new language becomes easy for the rest of your life.

My major concern is that this article is too one-sided: it mentions the difficulties/trade-offs of vegan diets, without mentioning difficulties/trade-offs of non-vegan diets. Eating a non-vegan diet is also not easy. Some examples of what you have to tell to people who want to eat animal products:

• Don’t eat too much meat, that is unhealthy. You can look on some websites how much gram per day is too much, according to your age and bodily needs.
• Fry the meat well enough, because (almost all) meat can contain harmful bacteria. Also wash well enough all the cutlery, the knife, the chopping board and everything that was in contact with the meat, because of contamination risks.
• But don’t fry your meat too much. Frying meat can produce carcinogenic substances. Especially when there is a dark or black crust visible, the meat was fried too much. For the same reason, avoid barbeque and flambé. Heating up meat in the microwave oven is not good enough to kill the bacteria. If you don’t know how to cook your meals properly, you can eat vegan meat alternatives: they can be safely eaten even uncooked (or used in the microwave oven).
• Don’t drink unpasteurized milk.
• Animal products don’t contain dietary fiber, so make sure to eat a proper source of dietary fiber. 
• Meat doesn’t contain vitamin C, so make sure to eat a proper source of vitamin C. There are websites that tell you which products contain vitamin C. 
• Avoid processed meat: that is unhealthy. You can look on some websites what counts as processed meat. I think bacon and ham also count as processed.
• Many meat products, especially fresh products, don’t show an expiration date on the package. You can look for some information on the internet how to learn to detect when your meat is expired. If you don’t know how to smell expired meat, don’t keep your meat too long in the fridge, or eat plant-based meat alternatives as they show a best before date on the package.
• In many cases, cheaper meat products may be unhealthier than more expensive meat products. There is often a trade-off between price and health/quality of the meat product. Consult a nutritionist to figure out the best diet according to your budget. 
• Almost none of the meat eaters had a recent blood test to check if they have for example too much bad cholesterol that could be the result of eating too much animal products. They also don’t know if their bodies can properly absorb for example the iron in the meat. It is recommended to visit a nutritionist and ask for a blood test. 
• Some people are allergic to milk, fish, and other animal products. You can consult your doctor if you don’t know about your potential allergies.

These examples should be enough to show that eating animal products is equally difficult as eating vegan. My worry is that focusing too much on vegan nutrition issues (telling people a lot about how to eat a healthy vegan diet), might give people the impression that veganism is difficult, and then they continue eating animal products and causing harm to animals. But focusing too little might be counterproductive as well, because then people don’t eat enough healthy vegan diets, they become ill and revert back to animal products. 

So I recommend that when you tell potential vegans how to eat a healthy vegan diet, you also mention the health concerns related to animal products, to make clear that eating a healthy non-vegan diet is equally difficult or easy. 

"But it’s hard to shake the feeling that farming cognitively disabled humans would be even worse than farming pigs." > I think this feeling is a moral illusion, comparable to an optical illusion where it is hard to shake the feeling that one line is longer than another. I wrote some articles about this: https://link.springer.com/article/10.1007/s11406-020-00282-7

https://link.springer.com/article/10.1007/s10790-015-9507-8 

And an infographic

https://stijnbruers.wordpress.com/2022/11/08/moral-illusions-infographic/ 

"in principle, it would be a good thing to farm short-lived happy humans (perhaps for their organs) who would otherwise not get to exist at all. But we find the idea repugnant, and that’s probably also a good thing. It causes us to lose out on some life-saving organs, and the value of the farmed lives themselves; but it may also prevent us from committing worse atrocities against each other."> I think people who have the strong moral intuition that it is wrong to use mentally disabled orphans as merely a means (as food, organs, experimental objects) do not believe that those humans are in fact not moral subjects but the main reason why we ought not to use those disabled humans is that we are too stupid to make a distinction between them and moral subjects, that we are not able to draw a line, that when we use them, we will also use moral subjects as merely a means.

About the idea of breeding happy beings to use them (e.g. happy slaves, happy farm animals): it is very difficult to justify this without stumbling upon very counter-intuitive conclusions. I wrote an article about population ethics and animal farming, arguing that happy animal farming is problematic and should also be avoided, even if the animals have net positive lives: https://www.pdcnet.org/enviroethics/content/enviroethics_2022_0999_10_26_45 

"It seems tragic for a human to be stuck with the cognitive capacities of a chicken—we feel that they’ve been deprived of capacities that they ought to have had. By contrast, it isn’t tragic for a chicken to have the cognitive capacity of a chicken.">Again, I believe this intuition that one thing is more tragic than the other, is a moral illusion, comparable to optical illusions. It is difficult to justify why one thing is more tragic. So, X and Y do not have property P, but the fact that X not having P is worse than Y not having P is because X looks more similar to Z who has property P? In what sense, and how similar? Or X has parents who have property P? Why parents and not cousins? That seems so arbitrary (comparable to arbitrariness behind optical illusions).

" If we possess a magic pill that would provide typical human intelligence to either individual, it seems we have stronger reason to give it to the cognitively disabled human than to the chicken (bracketing extrinsic factors, like how others would react)."> The only reasons I can think of, are arbitrary, and these are not strong reasons.

I think happy animal farming (breeding, killing and eating animals who had net-positive lives) is not permissible (except if the animal would be extremely happy). See population ethical arguments against happy animal farming: https://www.pdcnet.org/enviroethics/content/enviroethics_2022_0999_10_26_45

https://stijnbruers.wordpress.com/2016/11/05/can-we-eat-happy-meat/