If EMR could only operate standard weekday formations and were unable to bolster available stock, then they should have insisted only customers with seat reservations could travel on their Inter City services.
All customers without seat reservations would need to travel on EMR Connect services which should have been a mix of 8 and 12 car, or Thameslink services to Bedford.
But without proper planning and policing, customers will merely be sent by online Journey Planners via the quickest route, even if the St Pancras to Derby is a overcrowded 5 car and the Derby to Crewe an inadequate 2 car DMU. Customers like water will simply flow via the easiest or quickest route.
As for 5 car Aurora trains doubling up to 10 car formations, this would only be planned for the peak flow. Given the driving cabs are more expensive than a simple coach and will have less seating capacity, I'm surprised it costs less to manufacture and operate 2 x 5 car formations with 4 cabs, rather than 1 x 10 car formations with 2 cabs all day. There is also the additional cost of doubling of onboard staff as happens when LNER operate 2 x 5 car rather than 1 x 9 car.