I'm not entirely sure where the trains will stable at Redhill. I'm pretty sure there would be room to put them over in the loop/stabling point by the Tonbridge lines. Freight & engineering trains very occasionally use the facilities on the Down Side to wait for a possession to begin or for capacity through Redhill to become available, but there are also further overgrown sidings, just by the old brick huts & equipment rooms - it might be an idea to rebuild those. Put me down as "unsure", though!
As for the loops on the Up Side adjacent to Platform 1 - I don't believe they are "disused" in any official capacity, but I may be wrong. I'm pretty sure I've seen engineering equipment and defective wagons sitting there over the last year or thereabouts. They won't last for ever, though - that's the only place the Platform 0 enhancement can really go.
With regards to the siding/headshunt past the London end of the station, do remember that this is often used by RHTTs, particularly on driver training runs (as and when needed) but also at other times. As deicing is often needed in the area overnight, it would make sense for this area to remain available to all trains at short notice, and also to have dedicated sidings for passenger stock away from those footpaths next to the line beside the bridge (to avoid graffiti if the trains stay there for a number of hours).
I suspect that any new holding sidings would partially be used for the stock for the first train(s) of the day from Redhill, which can be stored either at Gatwick CHS or Redhill's headshunt/Long Siding (in the case of FGW) as required, but usually in the old Post Office bay or Platform 3 (in the case of Southern's 455 which runs ECS from Caterham prior to the start of service).