If I get on the DLR at, say Crossharbour, and present my Oystercard
and then get of the DLR at Heron Quays and change to the Jubilee line,
will I be charged for one journey or 2? I presume you are meant to
swipe at Heron Quays?

Secondly, As a prepay is initially cahrged at full fare, and refunded
on exit, what happens if there is a station evacuation? You will not
be able to swipe out so will be overcharged, I presume? (There are
many evacuations where there isn't any real imminent danger - just
precautionary - so don't say that people will be pleased to get out
alive and not worry about it :-) )