How many terminating platforms are they building at Paddington anyway?
Wouldn't it be better to tie all these branches into the core Crossrail service as ways to disperse the trains as is the plan with Thameslink and the GN?
EDIT:
THe Greenford Branch could retain direct service to Paddington via the New North Main Line which could be electrified as a diversion route and to help reduce the cost of providing backup terminus space for Euston and Chiltern in line with the latters expansion (EWR) and possible electrification.
Meanwhile the other branch lines already seem to have generally long station platforms that could be used with the planned 200m trains, or would be easily extendable, especially as its perfectly possible for the trains to be fitted with SDO as the tender hasnt gone out yet.
It would appear that a captive shuttle would be needed between Bourne End and Marlow due to the reversal not being long enough for a 200m train, this I would suggest would be some sort of 25kV powered Parry People Mover, unless anyone can think of a cunning way to fit a chord into that length.
EDIT #2:
Why would a Crossrail Train have to be fixed formation? Surely the electronics we have these days would be capable of communicating the length of the train to the platform controller which could then only open the relevant number of doors at the front of the platform? This information could be displayed on the Platform Information display, or perhaps PID screens mounted above the edge doors that show which doors will open for each train.