While washout typically means the wingtip has a lower angle of incidence than the root, it is not universal. However, I don't see this as the case with the screenshots shown elsewhere without the harsh shadowing that makes it difficult to determine the chord line of the tip in the screenshots here. I see a slight washout from root to tip. The problem is the wing/body fairing is huge and not proportional to the wing shape making it deceptive to the actual wing chord. However, the charts referenced before seem to indicate the tip will stall before the root, so it's also possible that indeed the washout is inverse to normal. Remember, the overriding desire of the North American team was creating the most maneuverable airplane possible, but also one with the longest range. This is why the laminar flow wing was so important and why so much was done to ensure that it was the overriding point of focus for maintenance in the field behind the engine. There were things done to the wing that were unique to the P-51 because it was also the only one using a laminar flow wing at the time.
Then I gather that it MUST have "inverse washout," as you call it, then. Would that explain why the Mustang always drops a wing in the stall, or is that just due to torque? I don't have the Spitfire, but I thought that the Spitfire has a more conventional washout with the tips at a lower angle of incidence. How does the Spit behave in the stall?