One of the problems with Marylebone was that most of the platforms were probably a bit short to cater for typical WCML train lengths, so passengers in rear coaches may have had to walk through several coaches before they could alight onto a platform.
I think Kensington Olympia was preferred for diversions when that option was available.
Quite so, although the photo I was looking at only shows the first seven coaches of the train coming under a bridge, but passing Wendover would almost certainly have put this train heading into Marylebone. Some of the diversions into Kensington used the GW/GC joint line from Grendon Underwood Junction via High Wycombe, Greenford Loop, Acton, Willesden SW and into Kensington.
I seem to recall there were a number of 'Starlight Specials' and one long distance Parcels service that started out of Marylebone in the early to mid 1960s while the West Coast Electrification work was being carried out.
From my own records on Saturday 27.01.79 the 04:05 Bletchley to Euston brought the wires down at Tring South Junction, blocking all four lines. That particular night 1M16, the Inverness and Fort William Sleeper, was running in two sections after the Fort William portion was heavily delayed by snow. The Inverness portion (7 coaches) was also running late, and was diverted via Birmingham New Street, Banbury, and High Wycombe into Marylebone. It was headed by a 47 (unfortunately didn't have the number). A 25 was sent over from Willesden and the 'top n tailed' formation went out to Neasden South Junction (reverse), Acton Canal Wharf Junction (reverse), and into Willesden via the Low Level Goods Lines.