What i'm wondering is why is it that when the Inertial Measurement Unit erroneous information was fed into the navigation system, and resulted in a negative estimated altitude, didn't the mars lander also have an accelerometer that could tell "hey, we're still accelerating, we're not stationary" and thus attempt some form of recovery of the navigation system.. perhaps reset the IMU or something? Reread data from the IMU after a certain delay?

I'm also quite confident that even before commencing entry, they had some idea from simulations as to how much the descent should take. Surely there's something wrong f you read negative altitude after just half the time required for landing.