If it does happen to be the unit in the picture (ie a Northern one), don't worry about fitting sound chips in all the cars - they often only ran with 3 engines working!
Re the pit lights: At Newton Heath, the pit lights are split into two, so either half of each road can be lit. I imagine it's the same at loco-only depots!
I would imagine that the ex-Royal Mail coaches have been bought purely for the bogies / wheelsets, although one or two may escape the totch to become store coaches.
It's looking great! Seeing the photo of the threaded bar supports makes your idea clearer now. An elegant solution to what could have been a mass of plywood and 2"x2".
There are some amazing layouts on RMweb and this is definately one of them. I have had a few holidays along the North Wales coast and the pictures of your layout bring the memories back!