"So how has Boeing and the FAA got around this conundrum ? What am I missing here ?" (Big Pistons Forever)
It was questions exactly like these which were the premise of the FOIA lawsuit against FAA. The point of that litigation was to force FAA to divulge the documentary record upon which it was proposing to act with regard to returning the aircraft to service. And more specifically, to divulge those documents so that a group of independent, distinguished experts and authoritative professionals could inform the public about whether FAA's proposed action would meet safety requirements. The disclosures, and the review, were intended to be complete before the return to service decision would be made final.
But FOIA litigation doesn't exactly move with speed requiring wind tunnel tests.