Both of your questions have been answered by eyewitnesses on the Railpage forum at the following web address.http://www.railpage.com.au/f-p1275998.htm
But lets clear up one rumour for once and all - the mail tunnel from Spencer Street did
go to the G.P.O. however in February 1963, when the tunnel was completed, the G.P.O. was on the corner of Spencer Street and Bourke Street (now the Mail Exchange).
From 1841 until 29 May 1917 the G.P.O. was the Elizabeth Street Post Office (on the corner of Elizabeth and Bourke Streets) and then from 30 May 1917 until 31 December 1963 the G.P.O. was the Spencer Street Post Office (on the corner of Spencer and Bourke Streets) and then again on 1 January 1964 until 21 September 2001 the Elizabeth Street Post Office was reinstated as the G.P.O.
In addition, mail trains didn't run under Spencer Street, they were railway platform trucks that ran through the tunnel.
Spencer Street mail tunnel 24 May 1962.JPG [ 223.31 KiB | Viewed 819 times ]
Exact date that the Spencer Street Station subway was closed to passengers is Saturday 28 May 2005.
File comment: Spencer Street Station subway closed 28 May 2005
Spencer Street Station redevelopment 2004 & 2005.JPG [ 37.02 KiB | Viewed 811 times ]