MickG0105, re
" I'm assuming that MCAS commands the AP motor. I'm not an engineer but that arrangement would seem to create the potential for an interesting battle between one motor driving through a single clutch and the other motor driving through the repeated engagement of an electro-magnetic clutch. "
This is a larger question concerning complex systems and their interactions in which there may be conflicting commands depending upon sensor input, (here, 'wrong' sensor data). Things are getting much more complex as computer systems invite a digital world to make use of data in ways that are not possible with mechanical systems. Determining all possible such conflicts (or software bugs) is no longer a realistic target but eliminating, or building a resilient system, (alternative solutions, back-ups, dual-dual systems, etc.), is a realistic target.
Examining this is a design and engineering responsibility, here, of Boeing's. This is apparently a system which relies upon one sensor (out of two or three), and so is potentially a single-point-of -failure. Such a system can be described as "brittle" rather than "resilient". So a question would be, was the testing for all possible failure modes done and if so, what were the results? They will be documented somewhere and so finding out is just a matter of time.
Understand the opinion that not every snag requires a test flight. That is entirely realistic and is normal for the industry. What can be argued is the need for a test flight when primary flight controls are involved along with a continuous stick-shaker and the caution messages that were experienced on the previous three flights, and this goes to communication when writing up snags, (as has already been observed here).
PJ2