The electrification to Hastings in the 1930s was indeed extended through to Ore because Hastings station is on a very tight site, and electric services terminated there from the start. There used to be a substantial EMU depot at Ore. There were some comparable ones elsewhere on the Southern Electric, stations like Addiscombe or Coulsdon North. Likewise the steam loco shed for Hastings had to be the other way, at St Leonards, which later became the diesel depot.
Regarding route knowledge diversions, that is a feature of privatisation; in times past there would be normal daily duties from a depot which covered all the routes and which came your way at intervals. Come separate TOCs, incompatible trains with incompatible controls, and all that, and it's not seen as possible even to swap things around to handle this. The GWR and the Southern used to have a couple of duties on normal trains on each other's lines between Exeter and Plymouth, and some other lines in the area (Barnstaple crews knew the way through Taunton right to Yeovil Junction), all specifically so they knew the alternatives.