I can't see them tunnelling under large buildings for insurance reasons.
The point of the DLR is that it can twist and turn around existing structures and routes. Not that I see much point in extending the DLR to Catford - that route is well served - unless the track is ultimately headed our way.

Lewisham is the nearest transport hub to FH, yet it can take an hour to get there by bus and there are no direct trains, so that would be a useful extension.