It would be a shame to go back on the First product idea for Manchester. TFW are in a position where reliability is improving and once all the new fleet is in they can really work on rebuilding reputation, which has been destroyed over recent years under TFW and Arriva.Reading between the lines leasing 67s for longer is effectively buying time for bi-mode/battery technology to improve and for (hopefully) a better financial position to start looking at replacement. Wouldn't be hugely suprised if some 197 replacement was factored in too with a then consistent product offering 1st and kitchen on every service, the displaced 197s can cause an internal cascade seeing off the 153s and 230s and capacity enhancement elsewhere. Heading firmly into speculation territory there though!!
As it stands like you say the money just isn't freely flowing, any withdrawal of Mk4s would almost certainly just see shortforms elsewhere and the Marches heading back to mostly 2 and 3 car 197s every hour.
How much life is left in the Mk4 coaches? Aren't they over 30 years old?
How much work would be required to get the coaches compatible with any new loco? If that isn't worth the time and investment, what other coaches would be available?