So, in the Lincoln assassination example, you could start the simulation at the moment you 'change history' because everything else that informs that moment is either present in your simulation (e.g., the temperature of the theater), or is in the past so actions on those items is informed by the participants memories. After that moment, I think you do need to simulate the whole thing. The reason is that the experiencing self makes decisions in the moment, and those decisions may inform the remembering self later.
And, of course, the whole thing rests on the nature of consciousness. If it's not a black box to the simulators they might be able to take shortcuts; or they may not have reason to generate consciousness at all if they know what consciousness will do in a given scenario.
For a full "ancestor simulation" yes, that's true.
Though does it have to be full, to look like reality does to us (e.g. for the simulation to be like what we live in)?
At some level, remembering X and having had X is the same with regard to the individual (especially if you can also make their organism not feel hungry, depleted, etc, the same as having had breakfast).
This reminds me of the "5 minute world hypothesis": "The five-minute hypothesis is a skeptical hypothesis put forth by the philosopher Bertrand Russell that proposes that the universe sprang into existence five minutes ago from nothing, with human memory and all other signs of history included".
This wouldn't be a "Bostrom-style ancestor simulation" that starts from the Big Bang say and lets everything unfold -- but it could still be a simulation that sees how such things as us, given the starting rules and conditions set at an arbitrary point in time behave.
I guess a problem with that is that it doesn't let our evolution/history fully freely unfold (from big bang and very limited initial configuration).
Then again, they could have had it "fully unfold", got to something like a "year 2500 A.D" humanity, and now are trying various changes at different eras to see what they get. In such a setup they can take shortcuts, because they know the tolerances of the system (and e.g. that a "fake full" stomach of Lincoln wont change anything of substance).