Surely the sensible thing to do is have the DLR and Overground extensions (to Dagenham Dock and Thamesmead, respectively) interchanging at Barking Riverside - that way you're only paying for one river crossing?
I think the problem with that idea is that the Overground is limited to 4tph because of clashes with freight further up the line. So you'd end up with - by London standards - a pretty mediocre train service for Thamesmead. (Though maybe you could run more trains as shuttles that terminate at Barking?). Also, being heavy-rail, it's not really suited to the close-together stations of the DLR, so would only have one station in Thamesmead. Since Thamesmead is pretty big, the one station isn't going to serve the entire town very well.
...and terminating the LO tracks at a dead-end station at Abbey Wood seems very wasteful...either of the other options would be better for the network as a whole, you would think, unless there was a plan for further extension beyond there. Abbey Wood already has Crossrail, after all.
True, but then it's not really for the benefit of Abbey Wood. It's for the benefit of Thamesmead residents, and I suspect they'd quite like any new line to go to somewhere that gives easy interchange onto Crossrail
To be honest, much as I would love the Overground to cross the river: Looking at the map TfL have put out, I'd expect the DLR to get the go-ahead and the Overground to stall. The proposed DLR route mostly runs along very wide dual carriageways with plenty of room to build elevated tracks and stations, and can give a very frequent service serving most of Thamesmead. By contrast, barring a LOT of property demolition, the overground route on that map would have to be mainly underground, as would the proposed Thamesmead Central station and (if it went to Abbey Wood) the Overground platforms there. *Very* expensive for much less benefit.