Before you read this, I've never been to Norwich so don't know whether there would be a platform or siding where the unit could stable for half an hour if it was on time, and these are just views from a twice daily passenger on this route - I'm not in the industry...
Sheffield would make the best sense. At the moment, platform 7 is only used occasionally by the very service were talking about, plus one of the EM to London in the morning.
The service from Manchester could terminate on 1A, meaning 1B is still serviceable for XC from the South to the North. It could work on a 5 hour round trip, 2 hours roughly to Lime Street, half an hour recovery, 2 hours to Sheffield, half an hour recovery.
The service from Norwich could terminate on 7, and because it's so rarely used, it means it can have lots of recovery time. Sheffield to Norwich takes 4 hours give or take, so the service could work on a 9 hour round trip - 4 hours to Norwich, half an hour recovery, service to Sheffield, half an hour recovery.
Platform 7 can take a 4 car 158 easily so can foresee no problems with that. As for 1A, I believe it can do 5 without looking at the sectional appendix.