My suggestion though, was not that a hydraulic or landing gear issue caused this, but simply that heat from a fire could cause a structural failure of the wing.
That's undoubtedly true, but I don't see how it fits in this case. The Mallard crashed immediately after takeoff so even had there been an engine fire it surely would not have destroyed the wing in what couldn't have been more than a minute or two. To me the video is sadly reminiscent of that of the P4Y breakup a couple of years back. Pure speculation (so spare me the finger-wagging !), but it looks to me like a structural failure precipitated the explosion/fire and not the other way round.