A very common common counterargument to improving the lots of various animals is that the animals aren't conscious.  A letter signed by experts seems useful in debunking, or at least casting doubt, on such counterarguments.  You are right to point out that this doesn't solve the difficult problem of comparing welfare or rights between species, but it's still a very important step.

Why would this be a distraction even if not very significant?  Seems one could sign the letter and argue for the position while doing all the things you like.  

I think DC's point is that the donation one can makes from the proceeds of selling the kidney outweighs the counterfactual direct impact of donating the kidney.

Perhaps "moral obligation" has a specific legalistic/Christian etymology.

This is one of the positions G.E.M. Anscombe defends in her influential essay "Modern Moral Philosophy".  She argues in part that the moral "ought" is a vestige of religious ethics, which doesn't make much sense without a (divine) lawgiver.  Indeed, one of the starting points of many modern virtue theorists is arguing that the specific moral sense of "ought" and moral sense of "good" are spurious and unfounded.  One such view is in Philippa Foot's Natural Goodness, which argues instead that the goodness ethics cares about is natural goodness and defect (e.g., "the wolf who fails to contribute to the hunt is defective" is supposed to be a statement about a natural, rather than moral, defect of the wolf).

I found this list very helpful, thank you!

On exotic tofu: I am not yet convinced that Stiffman doesn't have the requisite charisma.  Is your concern that he's vegan (hence less relatable to non-vegans), his messaging in Broken Cuisine specifically, or something else?  I am sympathetic to the first concern, but not as convinced by the second.  In particular, from what little else I've read from Stiffman, his messaging is more like his original post on this Forum: positive and minimally doom-y.  See, for example, his article in Asterisk, this podcast episode (on what appears to be a decently popular podcast?), and his newsletter.  

Have you reached out to him directly about your concerns about his messaging?  Your comments seem very plausible to me and reaching out seems to have a high upside. 

Although what you said might be part of the explanation for why many EAs focus on alignment or governance research rather than pause advocacy, I think the bigger part is that many EAs think that pause advocacy isn't as good as research.  See, e.g., some of these posts.  

 I haven't yet looked at the papers cited, but aren't they probably hopelessly confounded?  This seems to be one of the areas where it's hardest to measure causal effects.

Answers to this question could be relevant: What are some artworks relevant to EA?

Undoubtedly these are interesting questions, and I don't have much to contribute now.  Your thought experiment reminds me of Timmerman's Drowning Children case from "Sometimes there is nothing wrong with letting a child drown".  Timmerman argues with this case that we should reject the strong conclusion from "Famine, Affluence, and Morality".

I agree that the simple story of a producer reacting to changing demand directly is oversimplified.  I think we differ in that I think that absent specific information, we should assume that any commonly consumed animal product's supply response to changing demand should be similar to the ones from Compassion, by the Pound. In other words, we should have our prior on impact centered around some of the numbers from there, and update from there.  I can explain why I think this in more detail if we disagree on this.

Leather example:

Sure, I chose this example to show how one's impact can be diluted, but I also think that decreasing leather consumption is unusually low-impact.  I don't think the stories for other animal products are as convincing.  To take your examples:

• Eggs for human consumption are unfertilized, so I'm not sure how they are useful for hatching.  Perhaps you are thinking that producers could fertilize the eggs, but that seems expensive and wouldn't make sense if demand for eggs is decreasing.
• Perhaps I am uncreative, but I'm not sure how one would redirect unused animal products in a way that would replace the demand from human consumption.  Raising an animal seems pretty expensive, so I'm not sure in what scenario this would be so profitable.
• If we are taking into account the sort of "meta" effects of consuming fewer animal products (such as your example of causing people to innovate new ways of using animal products), then I agree that these increase the variance of impact but I suspect that they strongly skew the distribution of impact towards greater rather than lesser impact.  Some specific, and straightforward, examples: companies research more alternatives to meat; society has to accommodate more vegans and vegan food ends up more widespread and appealing, making more people interested in the transition; people are influenced by their reducetarian friends to eat less meat.

Voting:

I'll need to think about it more, but as with two-candidate votes, I think that petitions can often have better than 1:1 impact.

Not an expert, but I think your impression is correct.  See this post, for example (I recommend the whole sequence).