Most of course would look up 'Buying Uk train tickets' and any app will come up and most have a language changer tool anyway so however the visitor may find easiest is up to them.
Exactly, we have such rubbish (Only Gatwick and Horsham?) Southerly destinations as Sunday Brighton service would be most useful as you said for families who can (especially with children) can find Gatwick Airport stressful. I do still even by myself.
If Bognors and Portsmouths (Do most people use these for the south coast at Redhill anyway?) can go via Redhill why can't Brightons?

I think as there is 2tph VIC-BTN IIRC, 1tph should have calling pattern:
Victoria-Clapham-East Croydon-Redhill-Gatwick-Three Bridges-Hayward Heath-Burgess Hill-Hassocks-Brighton (same on return)
I suppose it doesn't necessarily need to be Victoria Originating services could be Bedfords/Cambridges?
And the other would go via quarry line bypassing Redhill as you know very well. Thought that one avoiding Redhill would be useful as at Gatwick London Bound, it may too busy for RDH passengers to get on.
I definitely won't be expecting them back anytime soon. Current services are great to get to London already. If You require Victoria but don't want to pay a higher fare for GatEx just go to Blackfriars and take Circle/District onwards.
Why? I am really lobbying right now for Thameslink but Victoria is so out of it? Tourists won't be looking for Clapham if so just change at Preston park or so. Thameslink is a good way for such tourists to get to London as you are plonked straight in the middle of it?