I appreciate point #2: "The probability that I exist, given that I exist, is 1."
However, the question addressed by the OP is not "What is the probability that I would exist in this world?" but "What is the probability, across the space of all possible continuations of the world from the dawn of humanity, that the world would end up in a state that includes me?"
But given that "me" includes all my experiences as well as my genetics, we have to throw the probability of all those into the equation as well.
At the very least, if you're going to take these sorts of calculations seriously you're going to have to ask what the probability is not only that I exist, but that everyone I've ever met also exists.
> "What is the probability, across the space of all possible continuations of the world from the dawn of humanity, that the world would end up in a state that includes me?"
Not particularly more or less probable than most particular states that don't include me, I imagine?
I don't understand talking about possibility when discussing the past. Probability reflects our uncertainty about what outcomes will happen. Uncertainty is a property of our understanding, not of macro reality. When looking backwards, we know what happened and what didn't, I happened (probability: 1) and those other worlds didn't (probability: 0). Saying "they could have happened" seems to me a sentence with no real meaning.
Even if you go with a many-worlds view, that only means that all things happened with a probability of 1.
However, the question addressed by the OP is not "What is the probability that I would exist in this world?" but "What is the probability, across the space of all possible continuations of the world from the dawn of humanity, that the world would end up in a state that includes me?"