As I understand it (and it depends on software reliability and certification), plan is to operate the West side services as 9car, the last TfL Board papers I read suggested a target of Mar-Apr 2020@samuelmorris + @JonathanH
1. Extra 7cars is short term measure till they get ETCS + some software / Heathrow Tunnels sorted at which point more 9car are needed, but this should be easily managed given the 9cars floating around OOC. It would be sensible to go 9car on the current 7car western routes get them bedded in then go for Heathrow branch to de-risk things a bit.
2. As far as it understand it the newly shortened 7car going east was to do with the Ilford being practised in bedding 7car units in reliably and getting milage in at quicker rate than out west. It also sent bedded in units out west which might highlight what are train vs infrastructure issues better and help with snagging.
3. With the big opening phases 2.5 years behind lots of 9cars needed for mid 2021 and Abbey Wood -Paddington opening as well as all the testing before hand (realistically 8-10 test units) after this there isn't much float as they are already running the Reading/Maidenhead routes with 345s which wasn't part of the original plan
The 315s are needed for the transition whilst Liverpool Street platforms 16-18 cannot take longer trains. I am not aware of any physical reason why they could not be parked in the sidings instead of 345s until this transition period starts, but maybe something in leasing arrangements is stopping a temporary substitution to 345s. Probably need to keep at least one pair of 315s in use to avoid staff lapsing on type training.
It appears the (revised) plan is to instigate the Stratford-Whitechapel section with summer timetable change and the Paddington connection 6 months later with December timetable change. Obviously the tunnel section is self contained so can start any date. I'm not clear why it needs to match National timetable change dates unless the Eastern and Western services are not currently operating to the through timings at the local stations (which I find hard to believe in West as virtually every train changed times in December 2019, although I could understand East timings not being final)