I agree that BML2 is possibly too ambtious. They just need to reopen the Lewes-Uckfield line (none of it has been developed on) and upgrade that route so there is a good alternative when the BML is down.
It's amazing calls for an alternative route have been consistently rejected. Even without BML2 just reopening the 7 miles of track between Lewes and Uckfield would provide a solution and would be a tiny, tiny fraction of the cost of HS2.