There seems no easy way to split the 158s from the Pacers, and if there was then the 158s would be more suited to running more/all of the fast services through Rochdale than similar "slow shuttles" in Lancashire/ Cheshire.
The Manchester to Airport/Alderley Edge/Crewe/Macclesfield/Stoke have to be operated by 90mph (or faster) stock except for a couple of services in total per day which can have 75mph stock.
If you want more than one EMU using the Hazel Grove electrics per day the only options than can be implemented until the Thameslink cascade are:
1. Use 158s on Northern's WCML Piccadilly services, releasing 323s.
2. Use any DMUs on Glossop/Hadfield services - then those overhead electrics will get limited use instead.
I suggested using 158s on peak time Alderley Edge services, but off-peak the 158s could be used on one of Airport-Liverpool, Piccadilly-Buxton or Piccadilly-Chester services and then switched for 156s in the peak periods.
On some services a three coach train would be fine from Scotland to Manchester Airport *if* it wasn't for the Preston - Bolton - Manchester passengers. Can't blame them for opting for the TPE services, of course, but it does put them under a lot of pressure (IIRC when Virgin ran the route some journeys had picking up/ setting down restrictions at Bolton)
Maybe you need to remind yourself of the May 2007 timetable where the standard hourly pattern between Piccadilly-Bolton-Preston was:
1tph Manchester Airport to Barrow/Windermere
1tph Manchester Airport to Blackpool
1tph Buxton to Blackpool
On top of that off-pattern was the Virgin Manchester-Scotland service.
Now the standard hourly pattern between Piccadilly-Bolton-Preston is:
1tph Manchester Airport to Barrow/
Scotland
1tph Manchester Airport to Blackpool
1tph
Hazel Grove to
Preston
There's no off-pattern additional services as there's extra Victoria services instead. It would be difficult for TPE to have Bolton restrictions on the Scottish services when that service between Manchester and Lancaster is now in lieu of a Windermere service.
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I expect most from Stockport would simply take one of the many trains to Piccadilly.
Have you seen on CrossCountry timetables where it recommends where possible you change at Wolverhampton to avoid changing at Birmingham New Street? That is what people do at Stockport, change there instead of at Piccadilly as it's quicker to make a connection. Adding extra changes or removing convenient connections between services will put people off travelling by rail - you only have to see what happened when XC removed the Stockport stop from some of their services, only to later add it back in.