If a single leg of a Liverpool to Newcastle journey leg is crewed by 3 drivers in each direction, it's therefore dependent on 6 crew being in the right place at the right time to make it work.
As you say, it leads to the timetable being so fragile as to not withstand any disruption at all, despite the diagrams and timetable being compliant with train planning and crew planning rules.
I think the significance of this issue affecting reliability over the last 5 years or so is under-analysed and therefore not yet understood, but it seems to have had a major impact on traincrew delays.
The attached chart, taken from ORR data linked below, shows the pattern of traincrew delays over the last 7 years.