With good signalling, 6tph on each branch, int each direction, should be no problem. The London Underground sub-surface have lots of level junctions and manage that sort of frequency, albeit with occasional delays and foul ups. The big advantage the line has over the DART is the commuter trains don't have to share with Intercity services.
I think everything in the plan has merit. I don't have a huge amount of hope that it will ever be built, but maybe some day.