Having been a design authority for safety critical systems, I can say that there are certain specific procedures that must be used.
When designing avionics, the airframe company (Boeing) would normally send a set of requirements (and those can be pretty onerous), but in this case Boeing did not ask a couple of questions that would have prevented this whole thing (at least if engineering had been in charge, which has not been true since the bean counters of McDonnell Douglas took over).
Question 1. Can this piece of kit move a flying control surface. The answer here is either yes or no. If the answer is yes (and here it most definitely was) then it is designated by default as a flight safety critical piece of equipment (level A in the industry terminology). That requires a minimum of 2 independent channels operating at the same time*. Now it is possible that the engineers said ''Yes it is flight safety critical" but were ignored / overruled by management. For that to happen it had to go pretty high in the chain.
When Boeing was still run by engineers (as it was when the 777 was being designed) they specified a triplex flight control computer architecture (there are many things to consider even here**).
A level A design can be moved lower but only if it can be thoroughly shown that other systems can stop it from being dangerous. Boeing said (in the documents they submitted) that the amount of control over the horizontal stabiliser was minimal but the final product (which was not reported to the FAA) had full travel control (albeit a few degrees at a time).
The stabiliser has far more pitch authority than the elevators and so when the worst happened, it would be impossible to recover the aircraft if the stabiliser could not be moved (which was impossible to do because of other design decisions). Because of that, a move from level A was completely unjustified. As I said on a thread at the time, relying on a single sensor and a single computer *** was utter madness.
Question 2. If the answer to question 1 was no (moot here but I will go ahead with it), can this interfere in any way with any critical system?
There are more, but you get the idea. If there was any true justice in the world the managers and executives who pushed this through should be having a very long holiday at the expense of the USA in a nice building such as Leavenworth.
* 2 channels is acceptable if the system can alert the pilot to a malfunction and 'get out of the way' so the pilot can take control. This is fine for some equipment but implies that the aircraft is still manually controllable.
** In multi-channel systems even the processors in each 'lane' must have different architectures. Even though they are synchronised so they can vote there is a possibility that a particular architecture has a bug on the microcode; having 3 different architectures makes the possibility of a bug in the microcode at the same point in the program infeasible.
*** The MCAS had two computers and two AOA sensors****, but only one of each was used on a given flight. The equipment that was not used on a given flight would be used on the next flight.
**** AOA sensors are notorious for failing (sometimes intermittently) and even though they are used in other systems, the likelihood of all of them giving the wrong reading at the same time is very low. Apart from that there are many other parameters being checked which would show up a dodgy reading.
Note: If Boeing subcontracted the actual electronics (highly likely as they do not make their own avionics apart from things such as 'future flight deck') then I suspect there would be some very interesting correspondence.