PEI - As you state, one of the very important starting points for any design effort is to clearly list the going in assumptions. Those include everything from failure rates and modes for input signals your system may be using to the failure characteristics of the equipment within your system to the expected human inputs if your system includes an operator interface.
In this instance with 737 Max MCAS I am sure that Boeing is taking this extremely seriously and carefully examining all of the assumptions that went into the original design. I am confident that the right experts within Boeing are working this issue in conjunction with certification authorities to determine if changes to the MCAS design, documentation, and/or training are needed and what those should be. The best path forward will be defined by those who know the system in the most detail so I will not speculate as to what particular changes in any of these three areas should be made.