In my current (fuzzy) thinking, it can be considered Pascal's Mugging if the probability is several orders of magnitude too low for you to even attempt to calculate, and your 'mugger' therefore needs depend on the payoff being large enough to try to outweigh in your mind the not actually calculated tininess of the probability.

Your scenarios 1, 2 and 4 fall in that category where I'm concerned. Your scenario 3 does not -- the payoff being limited to merely the global ecosystem means that I can try to evaluate the probability I assign to the systemic risk of GMOs, and evaluate the benefit vs the risk in a normal fashion. Other technologies that may increase existential risks because of weaponisation or accident (nuclear fission, AI, nanotechnology, advanced bioengineering) can be treated similarly to GMOs with normal benefit-vs-risks calculations, they're not Pascal's muggings.