My concern is mostly for edge cases, like stretches of two-lane highway where both lanes are going the same speed because of congestion. Is the car going to fluctuate its speed often, to try to isolate itself from neighboring cars? Or would it reduce its speed even further below the limit so that the left lane is constantly passing?

Keeping the left lane moving faster is probably wisest, but it's going to infuriate the drivers behind the autonomous vehicle, even though they're in the slow lane.

How far would a car go to avoid being rear-ended? Would a tailgater be able to cause the car to accelerate? At what point would the car stop trying to avoid an accident because it's safer to get rear ended than continue avoiding? When you put these things on the road, human drivers are going to start messing with them.