I think you may be getting a little muddled with the Jews returning in 523 BCE, Bart, unless I'm just not following you. However, this comment popped out at me:
As this is regarded as a letter to Bardiya it can only be placed between 2nd and 8th month of Acc Darius, 522BCE
(More probably between the 4th and 7th months.)
So Jeffro, if the Samaritans and neighboring peoples wrote a letter to 'Artaxerxes'/Bardiya, we have to consider:
- The time it took for them to hear of Cambyses' death. The Behistun inscription indicates July 522 BCE as the time of his death.
- The time it took for them to hear about Bardiya's universal acceptance as new king, said to also be July (Behistun).
- The time it took to send and receive the letters.
Let's say the neighbors knew who the new boss was very quickly, by mid-July. They would have to meet up and confer with everyone concerned (Ezra 4:7-10), compose and translate a letter, send the letter to (presumably) the Persian capital Susa, allow time for the Persians to dig around the archives for historical information on the Judean kingdom, allow time for the Persians to compose a reply, have it sent back to the local Samaritans' or neighboring peoples' officials and have it read out to them before rushing over to Jerusalem to implement the order.
Google Earth calculates Jerusalem to Susa by foot on modern roads is about 988 miles (1174 km). If they can travel by very fit horses at a lightning 60 miles per day, it would still take at least 16 days there and 16 days back - a day less if Bardiya resided at the fort where he was said to have been killed, somewhere near Behistun.
Bardiya was dead by the end of September. Soon after Jerusalem got the reply, he'd be history. His order to stop building could hardly be enforced long-term, so why did the Jews wait to finish repairing the city and its walls until (the next) Artaxerxes' 20th year - 78 years - which made Nehemiah depressed - especially if you believe Darius gave the go-ahead to rebuild the city?