The problem with digital control systems is that they are not continuous. Each if statement causes discontinuous operation. If the system were analog then testing would be easier. I'm not recommending analog systems - just pointing out the difficulty in checking digital systems - especially systems with deliberate discontinuous actions: if the velocity exceeds vsetpoint (even by a little bit) then pitch up etc.
I have heard that the code is not audited by the FAA but is qualified by number of "good" flight hours. The code is propriety.