How much rigor is enough to demonstrate airworthiness for defense avionics? Where do you draw the line?

There is generally a lot more leniency in the defense industry when it comes to adhering to DO or A(M)C 20-193 objectives. It should be noted, however, that interference can have serious impacts on execution time, and may be a performance-limiting factor, not simply a barrier to certification. Generally, military certification authorities may have their own interpretation of the certification objectives, for example the AA-22-01 (USAF) or “Defence Standard 00-970” (UK MAA, though this points to AMC 20-193), but the core activities required to demonstrate airworthiness remain the same.