I received an email from TfL today that all but confirms my suspicions about the station:
There are indeed some issues regarding the safety of the narrow northbound platform which must be resolved before the station can open. London Overground Rail Operations Limited, the operator of London Overground, have identified potential risks to passengers as trains pass at high speed. As the station is owned by Network Rail and is being developed by Westfield (the developers of the shopping centre), it is their responsibility to ensure that safety requirements are met.What's implied is that Network Rail and Westfield are (or were) happy with the station as it is. The "built too narrow" farce angle may make a good headline, but it appears there's a genuine dispute about whether or not the platforms are safe.
As to what progress there is and when it might open:
Please be re-assured that TfL and LOROL are in regular contact with both parties, with a view to resolving these matters as soon as possible. At the moment, frustratingly, we are unable to confirm an opening date. London Rail has designated a senior manager to monitor developments, and both the Project Director at Westfield and the Network Rail director with responsibility for this station are working closely with him to determine a solution.There's a tiny bit of hope something's happening. I had a look last weekend and nothing's changed. The current wording on TfL's project page is "due to open shortly".
(The picture is the station as shown on the shopping centre hoarding, where it has no platforms at all)
[Latest: The wall will be moved, taking at least four months]