At around 15:50 today, I made my way down from the Northern line to platform 9 at Bank, heading for Westferry. The next train shown on the board on the platform was a Woolwich Arsenal service in 4 minutes but after a few seconds of being on the platform, a member of DLR staff started making PAs that passengers heading towards Lewisham should head over to platform 10 for the next train which I and a few others did.
Is this much of a usual thing? I've only ever known platform 10 to be a terminating platform, though I only tend to use the DLR a handful of times a month at most. From what I could tell, the train was pretty close to Shadwell before it crossed over to the regular track which must've prevented any trains entering Bank during that time.
The DLR's moving block system enables bi-directional running/unusual train movements with relative ease. Under normal running your assumption is right in that P10 is normally dedicated to terminating services reversing via the shunt neck before re-entering service on P9. Should there be any issue negating the use of the shunt neck or P9, P10 can be used as a bi-directional platform at the end of a single line that as you described double up again East of Shadwell station.
The last time I was in The City my DLR train arrived in the departure platform from the tunnel (not the shunt neck), emptied arriving passengers before loading and departing. I asked the Train Captain why this was and they replied that there was a defective train in the arrival platform.
AIUI whilst uncommon, it's not unusual for trains to be run bang road in the event of any disruption on the platforms in an attempt to keep the service on the move.