Mis-fuelling does seem like a pretty obvious scenario, although the problem is that Jet A-1 is heavier than Avgas (.8 to .71), so it really should have got into the fuel lines pretty early in the piece. The other problem is that it should be very simple to establish what went into the wings, it should have been known within hours of the accident and yet CASA have said nothing about it. Unless, of course, the refueller is trying to cover his arse...