The only practical turnround point from the South is surely Gatwick Airport, just about, under today's reduced frequency timetable. East Croydon is out and, largely, London Bridge is as well. It would presumably a bold control decision to stop the Horshams and Brightons at GTW the moment there is disruption of eg. >10 mins in the central section.... but maybe that's what needs to be done?
From the Midland, there is West Hampstead and, from the GN, there is ... er... nothing South of Letchworth. (Could perhaps reverse a few over the flyover at WGC, now that Fordland only has 2tph all stations....but nice two face platforms to play with?)
So, we are back to the #1 issue with TL: since it cannot practically ever be stopped when in operation, all of the infrastructure and trains from Gatwick to West Hampstead/Hitchin have to be maintained in tip-top condition to stop faults occurring in the first place. That means 'tip-top', not the standard level UK rail normally runs to. The Victoria line manages it, mostly, and even runs more trains per hour (although of course the fallbacks here are simpler). This means broken rails, track circuit faults, signalling failures, power supply failures and (fortunately more rare these days) door and train failures simply have to be prevented in the first place. End of story.
As an occasional user of this route (and someone caught up in Monday's fault at 0645) and having been involved in its planning in earlier stages, despite a great deal of effort, we are not there yet. As you all know, there's simply no way that 24tph could happen at present (and, fortunately, the demand isn't there!)
PS I seem to recall that the Maidstone service is trying to solve another problem, ie. where to put the trains from this route as they become 12 car due to demand growth. Similarly for the Rainhams. Less of a problem now, so switching back to VIC and Charing Cross less of an issue...?