what's "it" in "it being virtually certain."

"it being virtually certain that there are three independent 1 in 1000 events required, or nine independent 1 in 10 events required, or something along those lines"

models of what, final probability of what?

Models of the world that we use to determine how likely it is that Eliezer will play a critical role through a FAI team. Final probability of that happening.

A billion is big compared to the relative probabilities we're rationally entitled to have between models where a series of very improbable successes is required, and models where only a modest series of modestly improbable successes is required.