Re cumulative probability calculations, I just copied the non-cumulative probabilities column from Ajeya Cotra's spreadsheet, where she defines it as the difference between successive cumulative probabilities (I haven't dug deeply enough to know whether she calculates cumulative probabilities correctly). Either way, it makes fairly little difference, given how small the numbers are.
Re your second point, I basically agree that you should not work on AI Safety from a personal expected utility standpoint, as I address in the caveats. My main crux for this is just the marginal impact of any one person is miniscule. Though I do think that dying young is significantly worse than dying old, just in terms of QALY loss - if I avoid dying of Alzheimers, something will kill me soon after, but if I avoid dying in a bus today, I probably have a good 60 years left. I haven't run the numbers, but expect that it does notably reduce life expectancy for a young person today.
My goal was just to demonstrate that AI Safety is a real and pressing problem for people alive today, and that discussion around longtermism elides this, in a way that I think is misleading and harmful. And I think 'most likely reason for me to die young' is an emotionally visceral way to demonstrate that. The underlying point is just kind of obvious if you buy the claims in the reports, and so my goal here is not to give a logical argument for it, just to try driving that point home in a different way.
Fair! I made a throwaway pseudonym I don't anticipate using elsewhere, but this seems like a reasonable criticism of the pseudonym choice.
To be clear, I work on AI Safety for consequentialist reasons, and am aware that it seems overwhelmingly sensible from a longtermist perspective. I was trying to make the point that it also makes sense from a bunch of other perspectives, including perspectives that better feed in to my motivation system. It would still be worth working on even if this wasn't the case, but I think it's a point worth making.