Track across grass seems straight
It is curious to me that the tracks across the grass from where the airplane apparently departed the runway pavement to the edge of the down-sloping bank seem to be very straight. That indicates to me that whatever forces caused the yawing moment resulting in the deviation from runway centerline were active only when the airplane was on the runway and did not continue as it crossed the grass. If the problem resulted from stuck nose gear steering or asymmetric thrust I would think that the track across the grass would be curved. It is possible, however, that turning side force from a stuck nose gear would not continue onto the soft margin if the nose gear stopped rolling as it dug into to the dirt. Even if that were the case, I would think that an airplane departing the runway with any significant yaw rate would exhibit continued yaw motion (and thus curved tracks) during at least the first part of its motion off the edge of the runway.
I sure hope those data and voice recorders are released so that we can learn from this incident and reduce the likelihood of a repeat.