Philip
On Moderation
The TfN Train Service Planning paper confirms that TPE is only offering the split stopper (and the interworking at Manchester Airport) as a temporary expedient, ending at the December 2019 timetable change:
https://transportforthenorth.com/wp-content/uploads/Item-5-Train-Service-Planning.pdf
Reading between the lines, this seems to imply a possible further major recast of the TPE North timetable in December 2019. And maybe retention of more 185s and renegotiation of the premium payments that the Franchise Agreement requires First Group to make over the remaining years of the franchise.
TPE may not be in a position to decide to keep more 185s than originally planned, if another operator has already made a deal for the 22 due to come off lease. There is the upgraded Hope Valley route to think about; this will probably need the remaining 22 185s as a minimum in order to provide enough capacity for passengers.