Whatever Omega is doing that might kill you might not be tied to the mechanism that divides universes. It might be that the choice is between huge chance of all of the yous in every universe where you're offered this choice dying, vs. tiny chance they'll all survive.

Also, I'm pretty sure that Eliezer's argument is intended to test our intuitions in an environment without extraneous factors like MWI. Bringing MWI into the problem is sort of like asking if there's some sort of way to warn everyone off the tracks so no one dies in the Trolley Problem.