My own suggestion should Lewes - Uckfield go ahead would be for NR to broker a deal between the Lavender and Bluebell Lines whereby NR would pay to restore the Ardingly branch and move the Lavender operation onto that branch.
NR would then have the Lewes - Uckfield in full as a normal railway, the Lavender people would get a line in tip top condition with an NR connection at one end and a Bluebell connection at the other end. The Bluebell would have a partner line with which resources could be shared and would finally get the junction fully operational.
Clearly, however, there would need to be some sort of through ticketing to allow visitors to use both the Bluebell main and the ardingly Lavender operation which would mean the creation of some kind of mini-ORCATS for the two lines to thrash out their fair fare shares...