If Earhart's strongest signal was heard at Itaska at a very strong, S5, I would think it would be line of sight, within the 42 - 82 nm range suggested, rather than sky wave.
I suggest this, because a sky wave signal, would have had to travel some 500nm (round trip to the F-layer), and be substantially attenuated. Also, early in the morning, the D-layer of the ionosphere would be becoming ionized, providing some further attenuation.
Does this make any sense to anyone?