What do you mean by "apparent mind-killing properties of this proposal"?

Mind-killer. Saying that some people are going to have special privileges after the essentially taking-over-the-world enterprise goes through is a political statement.

This might be a real-life version of counterfactual mugging or trading across possible worlds.

I don't think it is. The donors can't make their decisions depending on whether the promise will actually be kept, they don't have Omega's powers. The only thing to go on here is estimating that it's morally right to heed such an agreement, and so FAI will factor that in.