>They failed to identify that the sensor became safety critical and should thus be subject to such requirements.

Whistleblower testimony indicated it wasn't a failure to identify it as safety critical, but a conscious decision not to mention it as such to the regulator, and not implement it as a dual sensor system as doing so would have caused the design to require Class D simulator training; which Boeing was relying on the abscence of as a selling point to prevent existing airlines from defecting to Airbus.

>They sold configurations with redundant copies, which were purchased by most high-end airlines, but they failed to make it mandatory due to their oversight and purchasers decided to cheap out on sensors since they were characterized as non-safety-critical even if they were useful and valuable.

Incorrect. All MAX's have two AoA vanes, each paired to a single Flight Computer. The plane has two Flight Computers, one on each side of the cockpit, and the computer in command is typically alternated between each flight. One computer per flight will be considered in-command (henceforth referred to as Main), the other will be henceforth referred to as operating as "auxillary". The configuration you're thinking of is an AoA disagree light, implemented by enabling a codepath in software running on the Main FC whereby a cross-check of the value from the AoA vane networked to the auxillary FC would light up a warning light to inform pilots that system automation would be impacted, because the AoA values between the MFC and AFC differed. A pilot would be expected to recognize this as and adapt behavior accordingly/take measures to troubleshoot their instruments. Importantly, however, this feature had zero influence on MCAS. MCAS only took into account inputs from the vane directly wired to the Main FC. While a cross-check happened elsewhere for the sole purpose of illuminating a diagnostic lamp, there was no cross-check functionality implemented within the scope of the MCAS subsystem. The MCAS system was not thoroughly documented in any delivered to the pilot documentation. The program test pilot got specific dispensation to leave that out of the flight manual. See the Congressional investigation, final NTSB, and FAA report.

>The manual, which pilots actually read, has instructions on how to disable the automatic tuning and enable redundant control systems and such procedures were correctly deployed at least once if not multiple times to avert crashes in premier airlines.

The documentation, which included an Airworthiness Directive and NOTAM, informed pilots any malfunction should be treated in the same manner as a stabilizer trim runaway. Said problem is characterized in aviation parlance as a continual uncommanded actuation of trim motors. MCAS, notably is not that. It is periodic, and in point of fact, it ramps up in intensity over time until over 2° of travel are commanded by the computer per actuation event, with the timer between actuations being reset to 5 seconds by use of the on yoke Stab trim switches. This was ncommunicated to pilots. Furthermore, there were design changes to the Stab-Trim Cutout switches between 737NG (MAX's predecessor), and MAX. In the NG, the Stab Trim cutout could isolate the FC alone, or both FC and yoke switches from the Stab Trim motor. In MAX, however, the switches were changed to never isolate the FC from the Stab trim motors, because MCAS being operational was required for being able to checkmark FAR compliance for occupant carrying aircraft. So when that cutout was used, all electrically assisted actuation of the horizontal stabilizer became unavailable. The manual trim wheel would be the only trim input, and in out-of-trim attitudes, would result in such excessive loading on the control surface that physical actuation without electronic assistance was not feasible on the timescales required to recover the plane. There was a maneuver known to assist with these conditions (when they occurred at high altitude) called "roller coastering" in which you dive further into the undesired direction to unload the control surface to render it actuable. This technique has not been in official documentation since Dino 737 (Pre-NG). The events you're referring to when uncommanded actuations were recovered on other flights, happened at high altitudes, and were recovered with countered electrical stab switch actuation followed by Stab trim cutout within the reset 5 second watchdog timer prior to MCAS activation subsequent to a Stab-trim yoke control switch actuation. This procedure, and the implementation details needed to fully understand its significance, were undocumented prior to the two crashes. Furthermore, this procedure to cut out MCAS/the MFC from the stab trim motor and finishing the flight in a completely manually trim controlled configuration meant that technically you were flying an aircraft in a configuration that could not be certified to carry passengers when taking the FAR's prescriptively, and uncompromisingly rules-as-written with zero slack offered for convenience, because MCAS was necessary for grandfathering the MAX under the old type cert, and without MCAS functional, it's technically a new beast, which is non-compliant with control stick force feedback curves when approaching stalls, which by the way, just to make it clear, a compliant curve has been a characteristic of every civil transport in all jurisdictions worldwide for well over 50 years.
This was not documented and only became apparent after investigation. Again, see the House findings, FAA report, and NTSB.

>Only a combination of all of those failures simultaneously caused fatalities to occur at a rate nearly comparable to driving the same distance, how horrifying!

Oh, the multi-billion dollar aircraft maker built a machine that crashes itself, gaslit it's regulators, pilots, airlines, and the flying public to juice the stock price so executives could meet their quarterly incentives, and diverted tunds away from it's QA and R&D functions to do stock buybacks, move HQ away from the factory floor, and try to union bust. With over 300 direct measurable deaths within a couple of months and multiple years worth of grounding and mandated redesigns to fix all the other cut corners we've been unearthing, and veritable billions of dollars of loss incurred in delays. Heavens, it could happen to anybody. How could you possibly see this as something to get upset about? /s