For instance, it seems that there is only a small difference between the happiness of richer nations and poorer nations, while the first consume many more resources than the second. Hence to increase utility we should simply kill off all the rich, and let the poor multiply to take their place (continually bumping off any of the poor that gets too rich).

This empirical claim seems ludicrously wrong, which I find distracting from the ethical claims. Most people in rich countries (except for those unable or unwilling to work or produce kids who will) are increasing the rate of technological advance by creating demand for improved versions of products, paying taxes, contributing to the above-average local political cultures, and similar. Such advance dominates resource consumption in affecting the welfare of the global poor (and long-term welfare of future people). They make charitable donations or buy products that enrich people like Bill Gates and Warren Buffett who make highly effective donations, and pay taxes for international aid.

The scientists and farmers use thousands of products and infrastructure provided by the rest of society, and this neglects industry, resource extraction, and the many supporting sectors that make productivity in primary and secondary production so far (accountants, financial markets, policing, public health, firefighting...). Even "frivolous" sectors like Hollywood generate a lot of consumer surplus around the world (they watch Hollywood movies in sub-Saharan Africa), and sometimes create net rewards for working harder to afford more luxuries (sometimes they may encourage leisure too much by a utilitarian standard).

Regarding other points:

fact that you should follow a utility function in no way compel you towards total utilitarianism

Yes, this is silly equivocation exacerbated by the use of similar-sounding words for several concepts, and one does occasionally see people making this error.

interpersonal utility comparisons (IUC)

The whole piece assumes preference utilitarianism, but much of it also applies to hedonistic utilitarianism: you need to make seemingly-arbitrary choices in interpersonal happiness/pleasure comparison as well.

When considering competing moral theories, total utilitarianism does not "win by default" thanks to its large values as the population increases.

I agree.

The most compelling argument for total utilitarianism (basically the one that establishes the repugnant conclusion), is a very long chain of imperfect reasoning, so there is no reason for the conclusion to be solid. Considering the preferences of non-existent beings does not establish total utilitarianism.

Maybe just point to the Stanford Encyclopedia of Philosophy entry and a few standard sources on this? This has been covered very heavily by philosophers, if not ad nauseam.