I don’t think your argument against risk aversion fully addresses the issue. You give one argument for diversification that is based on diminishing marginal utilities, and then show that this plausibly doesn’t apply in global charities. However, there’s a separate argument for diversification that is actually about risk itself, and not diminishing marginal utility. You should look at Lara Buchak’s book, “Risk and Rationality”, which argues that there is a distinct form of rational risk-aversion (or risk-seeking-ness). On a risk neutral approach, each outcome counts in exact proportion to its probability, regardless of whether it’s the best outcome, the worst, or in between. On a risk averse approach, the relative weight of the top ten percentiles of outcomes is less than the relative weight of the bottom ten percentiles of outcomes, and vice versa for risk seeking approaches.
This turns out to precisely correspond to ways to make sense of some kinds of inequality aversion—making things better for a worse off person improves the world more than making things equally much better for a better off person.
None of the arguments you give tell against this approach rather than the risk-neutral one.
One important challenge to the risk-sensitive approach is that, if you make large numbers of uncorrelated decisions, then the law of large numbers kicks in and it ends up behaving just like risk neutral decision theory. But these cases of making a single large global-scale intervention are precisely the ones in which you aren’t making a large number of uncorrelated decisions, and so considerations of risk sensitivity can become relevant.
The keywords in the academic discussion of this issue are the “Archimedean principle” (I forget if Archimedes was applying it to weight or distance or something else, but it’s the general term for the assumption that for any two quantities you’re interested in, a finite number of one is sufficient to exceed the other—there are also various non-Archimedean number systems, non-Archimedean measurement systems, and non-Archimedean value theories) and “lexicographic” preference (the idea is that when you are alphabetizing things like in a dictionary/lexicon, any word that begins with an M comes before any word that begins with a N, no matter how many Y’s and Z’s the M word has later and how many A’s and B’s the N word has later—similarly, some people argue that when you are comparing two states of affairs, any state of affairs where there are 1,000,001 living people is better than any state of affairs where there are 1,000,000 living people, no matter how impoverished the people in the first situation are and how wealthy the people in the second situation are). I’m very interested in non-Archimedean measurement systems formally, though I’m skeptical that they are relevant for value theory, and of the arguments for any lexicographic preference for one value over another, but if you’re interested in these questions, those are the terms you should search for. (And you might check out PhilPapers.org for these searches—it indexes all of the philosophy journals that I’m aware of, and many publications that aren’t primarily philosophy.)