And from me. It's egregious when hundreds of millions are being discussed in regards to Windmill, that these paths exist and are so underused.
Everything out of Victoria (fast) should call at Clapham Junction and East Croydon. Yes people use these trains for short hops, but even Gatwick is only 30-35 mins, and frankly at a high enough frequency, it's all spread out well enough. As I mentioned on another thread, stations with integrated 'normal' airport services like Amsterdam, Copenhagen and Zurich (vs. special dedicated trains) work due to good frequencies.
Of the four paths:
2tph as today: Vic - CJ - EC - Gatwick - HH - Brighton.
2tph could run: Vic - CJ - EC - Redhill - all stations to Gatwick and Three Bridges. This would restore the fast Redhill service and serve the intermediate stops which it sounds like the Reigate might displace. This fast Redhill could be the service to connect with the Tonbridge too.
If that isn't possible, same but 2tph to Hove. They can alternate Three Bridges and Haywards Heath stops, with Burgess Hill in peaks. Make the whole core/spine high frequency and 12 car throughout.
Fast forward - if Windmill is on the backburner like many capital projects will be, an audit of paths and short-forming will have to happen. It may be that more services can terminate at Gatwick, Three Bridges or Purley from the south. Especially as there is already the Victoria/LB split.