To clear up a few things:
There has been extensive testing on both fringes, with relative success. The failure of NR to deliver ETCS (originally planned for 2017) has a big impact as the system is designed to transition to ETCS.
There is no plan to commission ETCS L2 on GW before or during the October 20 to March 21 opening window (this is by NR/Alstom)
Both fringes have to be completely commissioned to open an abbey wood to Paddington service as the two depots are Ilford and OOC.
Trains can run end to end reliably, but the system has many complex interfaces and functions and it is these which take a lot of testing and software revisions.