It was suggested somewhere above that until re-modelling takes place the electrification from Victoria will only go into a bay platform, thereby avoiding putting wires above the tracks that will move during the re-modelling. The same surely applies to the tracks from Guide Bridge, so the earliest they would be electrified would be after completion of the new layout.
With the exception of some future EMUs terminating from Victoria (which therefore can't approach from Guide Bridge), all trains at Stalybridge will be diesel or bi-mode until the route is electrified at least as far as Leeds. So diversionary routes don't need to be electrified at least until that time, and possibly not until electrification of most of the northern destinations plus Calder Valley and other diversionary routes means most of the TPE fleet can be electric only.