I guess Webb chose TB for the main marshalling yard because of available space, lots of flat land, and the fact that it was already the junction to the Murray lands lines. There could also have been labour issues, better to start with a clean slate. There is land west of Murray Bridge but it is not flat. Because trains would be held there the marshalling yard would have need to be flat so up journeys would have to begin by backing down to Murray Bridge station area.
Murray Bridge loco facilities were antiquated; there is a wonderful series of pictures of an S class jacked up on a pigsty of sleepers having the 6'6" driving wheels changed out. The turn table was tiny, it still is extant at the western end of the yard.
Murray Bridge refreshment room building is still extant as is the station building. I am not sure if the refreshment rooms were railway operated or leased out.
A large loco, 500 class, would still be required to lift the down train out from the river crossing.
Not all trains were passenger trains and most of these were broken down at TB as you proposed. In the days of steam there were no through freight to Melbourne like the "Jets" which came with the diesel era.