(A) I agree 'astronomical observers' is not the same as 'astronomers'. I would also say that the wording of "the crucial event" is a little too extreme. It was a significant milestone in terms of the scientific community considering asteroid satellites, but "THE CRUCIAL event"?
(B) Pretty much. The theoreticians long believed it was THEORETICALLY possible, whereas this says that "observation supports the existence". But you're right that astronomers USED to be more skeptical and NOW they're more convinced.
(C) Perfect!
(D) What new THEORETICAL developments were in the passage? We always thought, theoretically, that asteroid-satellite systems were possible. New OBSERVATIONS, such as Herculina and the avalanche of similar findings that followed, have now left the scientific community thinking, "Hmmm, maybe so?" I would also be scared with how (D) uses the word 'respectable' from line 36, but in a completely opposite context. Line 36 was saying that BELIEVING in these systems is becoming respectable, while (D) is saying that DOUBTING these systems is respectable.
Nice work, though. I'm just offering you my take on them.