Re. the last point, I will also admit to being confused about the correct reference class to use here. Even if (purely hypothetically) we had a reason to guess that alien AGI had a decent chance to not only implement an acceptable morality according to their makers, but also be supportive to humanity by our morals ... well, if one of them was already here that would tell us something, but it would also put us into a position where understanding our own timeline to homegrown AGI development suddenly became much less important.

Which suggests to me it might still be a bad idea to just use that observation as direct input into our probability estimates, since it would bias the estimate in the class of cases where we really care about the accuracy of that particular estimate.