Criminal negligence seems to me to be more appropriate than manslaughter. They disabled safety features and replaced them with their own which failed to function correctly. They whole testing program seems to have been rushed and corners were cut.
Why don't we instead start with the individual who was supposed to be monitoring the road and the vehicle's systems? It's well known at this point that they in fact were negligent in their duty.
One small mistake, on something so critical, can ruin many peoples' lives.
The Texas version requires recklessness, and I think that's met here.