For the diversions, northampton/weedon is compulsary. Routes at Rugby/Hanslope can't be queried. The rest I would guess would be west midlands via stechford, bescot, bushbury to Stafford, Colwich to Crewe via Stoke/Kidsgrove and maybe via man picc, however, doubt they will ever get diverted via picc as it's full.
Also as they will have so few drivers, and the diversionary routes will be very rarely used, keeping competency up will be a big challenge. personally i'd say Stafford to Rugby via west midlands would be the most accessible and beneficial during times of disruption. They would be better putting an agreement in place with an FOC for a route conductor provision during times of engineering or disruption.