Many years ago, I worked with a man who had worked in the Nottingham Divisional Manager's Office prior to spring 1966, and I remember him telling me that the possibility of connecting the MML to the Derby Friargate - Nottingham Victoria line was once considered as a way to relieving pressure at Trent; the location of a connection at Derby was obvious, but I can't remember him explaining where the connection would have been at the Nottingham end, but there was an implication in the way he talked that trains would have used Midland and not Victoria.
There was also an article in Modern Railways - possibly it was still called Trains Illustrated at the time - regarding plans for the GC Main Line after the expresses were withdrawn in 1960, and I recall it talked about building new connections and using it primarily for express parcels trains; certainly, pre-Beeching there didn't seem to be any plans to completely close the line. And didn't the original proposals for the [Nottingham] Victoria Shopping Centre include a twin track tunnel on (I think) the east side of the site?
Back in about 1960, Trains Illustrated/Modern Railways reported that when the Marylebone suburban services were being recast when the class 115 DMUs arrived, some Aylesbury services were going to be extended north to Woodford Halse; did that happen? Apart from the daily return Nottingham semi-fast, I can't remember seeing any photos of 115s north of Aylesbury, with the stoppers north of Aylesbury remaining steam hauled until their withdrawal.
If I get the opportunity over the next few days I'll try and find the relevant article about BR's 1960ish plans for the GC Main Line; but certainly, it wasn't envisaged that it would close completely
Not sure Marylebone would have coped with all the East Midlands traffic on top of everything it currently deals with, nor that it's approaches could have been as easily upgraded to provide the necessary capacity.
Regarding the actual station, Marylebone had four platforms, and two could have been added where the cab road was, as has actually happened; land on the west side of the station was also owned by BR and occupied by office blocks, so there was some scope for expansion.
I think you make a valid comment about the approaches