Yes, I agree, it is non-trivial to assess (approximately, probably) bounded downside and this needs to be done on a case-by-case basis, including in global health.
However, if you find that there are fat tails on a portfolio level but none on the individual intervention level, then the individual intervention was probably not assessed comprehensively? Interaction effects and second order effects matter, of course, but at least a portion of them should arguably be credited back to individual iterventions as you assess them.
An important (and to me fairly open) related question is to what extent this ends up being action-guiding?
Eg if I lower my probability estimate of this materialising by 10 percentage points, how much will it affect resources I spend on helping to prepare for the possible outcome? Perhaps people here have thoughts on that—my impression is that working on improving the future opportunity set for such donations is relatively robustly good right now?