The Bluebell Railway owns the alignment and plans to reopen it as a Third Rail Heritage Line, to give them a place to run the 4VEP without a diesel or on the mainline.
Electrifying Ardingly has been half heartedly suggested for the last 15-20 years. It won’t happen. There are so many reasons why it would not be practical, not least having brand new live third rail. The same rules apply as with the mainline, being a light railway does not change that.
I have never seen a serious suggestion that the BRPS intend to put live third rail down. I have seen some suggestions of laying dummy third rail at Horsted Keynes for aesthetic reasons, and there are various ideas how the 4VEP and section to not quite Ardingly station might be operated.
If Ardingly ever gets off the ground (take a look at the railway’s finances to see the likeliness of that) the most practical of the suggestions are to fit the VEP with batteries and have a charging system at Horsted Keynes, or to simply run the VEP push-pull. There’s a handy 73 in the same livery now…
Going back to the whole diversionary route thing, I personally do not see the point of the second connection at Ardingly/Copyhold just to have a diversionary route. Realistically it just does not work, and the current diversions, whilst not perfect, do not involve traipsing over a heritage railway at 25mph and hired in Class 37s or something similar. Yes, it would be a unique selling point for railtours etc, but the cost for the railway in the current financial situation to lay an extra two miles of track to vaguely near a small village and then build a tunnel around a stone terminal…
The most logical new line in that area would be to connect Lewes and Uckfield via Ringmer and Isfield. Bringing a rail connection back to Barcombe (which has far fewer people living there than Ringmer) does not seem justifiable. That route would connect thousands of people together, rather than reopening East Grinstead - Copyhold Junction to serve a handful of people. All the stations on that line (with the exception of West Hoathly) are inconveniently sited in the middle of nowhere with only a few houses (often railway cottages) nearby.
Just because a railway never served it, doesn’t mean it can’t.