I don’t think P2 can be justified by Dutch Book type arguments. I don’t think it can be justified, as a rational requirement of choice, at all. My reservations are similar to Edward McClennen’s in “Sure Thing Doubts”.
So you would argue that, knowing a fact, your preferences can depend on what would have happened had that fact been false?
Right. (Not my preference necessarily, but a rational person’s.) The facts in question include past actions, which can form the basis of regrets. The value of an event can depend on its historical context—that doesn’t seem unreasonable.
Would we be able to write the outcomes as full histories?
I don’t see why not. However, I haven’t seen many (any?) decision theory treatments that do so.