What can we learn from the many account of Agamemnon's homecoming from the Trojan War?
As you state, there are many different account of Agamemnon's homecoming from the Trojan war, even if the most famous account come from the great, Greek tragedian, Aeschylus. The very fact that there are multiple version suggests a few things.
First, it shows that there are many variations of myths in the ancient world. This is an important point to consider, because there usually is not one standard version. Timothy Gantz, a good scholar of Greek mythology, has produced a two volume on the variations of Greek mythology. This would be a great book to consult to see the development of mythology.
Second, the differing myths also tells us something about Greek society. Greek society was not monolithic. There were many Greek cities (poleis) and they could differ greatly. And their stories could also greatly differ.