This is really a fairly complicated issue. I am not a Boeing apologist as they should have emphasized the MCAS more in their documentation and training. The training for use of the MCAS is similar to the training for the failure mode of runaway trim, a failure that is routinely trained for by airline crews. This is where the electrical trim system for the aircraft elevators fails and excessively trims the aircraft to either climb or dive uncontrollably. Usually, the fix is to pull the circuit breaker for the trim system, powering it down, so the aircraft can be properly trimmed manually.

Since the MCAS system is basically a trim system operated by the computer to trim the aircraft to avoid dangerous aircraft attitudes in certain flight regimes, it also has a circuit breaker and can be disabled at any time in flight by that means. The failure modes of runaway trim and MCAS are very similar with the aircraft elevators being incorrectly deflected. Had either of the crews in the crashes treated this as a runaway trim emergency the accidents may not have occurred.

Boeing's biggest fault was their failure to emphasize this training and have a single point failure mechanism in the MCAS angle of attack sensor feeding the air data computer. The FAA also shares some of the responsibility since they were aware of the MCAS system and apparently agreed with Boeing's decisions regarding the system and its operation.

I am not sure how they can blame the test pilot as they usually work for and report to an engineering executive manager who reviews the flight test data and either approves of or disagrees with the results. Unless the test pilot was falsifying these reports I am not sure how he could be held responsible.

But then, as we have often seen, in our legal system anyone can be sued for anything.

Training pilots would cost airlines money.

Blaming the test pilot got corporate Boeing off the hook. Maybe the test pilot designed MCAS himself and sneaked it into the plane without Boeing knowing about it. When big corporations are involved, they and the government make their own rules. No matter what kind of disaster takes place and how many people are killed, corporations seldom suffer any penalties, and certainly not the people running those corporations. If anyone takes the heat, it's a secretary or a low level supervisor.

Interesting development in the Boeing MCAS mess. The chief prosecutor in the Boeing case worked out a deal where Boeing would get off the hook for the two crashes that killed 346 people. Shortly after this decision, she left the Justice Department and joined Boeing's law firm. Now, if Boeing is sued again, they'll have an experienced lawyer to defend them. Still, someone had to be held accountable, so they're prosecuting the test pilot.

In case MCAS doesn't sound familiar, it's "secret" software that Boeing installed in the 737 Max so pilots wouldn't have to be retrained to fly this new variation of the 737. The Airbus A320 is serious competition for the 737, so in making the Max, Boeing didn't want to tell potential customers that pilot training would be required. Instead, they added MCAS, a software enhancement that would compensate for the changes made with the introduction is this new variant with its more powerful engines. Unfortunately, with the pilots not knowing there was software flying the plane, two of them crashed, and the 737 Max was grounded for twenty months.

There's lots of information available about this online.