Honestly, I really doubt we will ever have a credible investigation on this one.
Frankly this already looks like CFIT whilst likely flying outside commercial flight safety procedures. Given that this is a major new-model aircraft (huge investment, big sales push) the obvious conclusion looks like the one least prejudicial to the vested interests. Thus they would be idiots not to make a big show of this being an impartial and transparent investigation. It carries a small risk of evidence leading in an unexpected direction, but less risk (given appearances so far) than if they seem to be hiding something. That could kill the whole project.