Never expected all of that
And sorry to rake through it if the embers were dying away
There seems to be quite a few differing thoughts and quite a lot of repetition and just my thoughts but why do the MAA have to prove airworthiness? EASA or the FAA or the CAA do not have to prove airworthiness. The type certificate holder has to prove this by presentation of verification artefacts that demonstrate that a standard, (set of standards) has been achieved, compliance.
The CS and FAR are the framework to be verified against or by presentation of equivalent safety findings to argue what has been done is as good as those standards. EASA and FAA may mutually recognise each others work by validation but this considers that CS and FAR are very close in content and intent.
In the absence of those artefacts or mutual recognition based upon accepted standards the air vehicle has unknown provenance so who decides it is safe and where and when to fly it? EASA would say not happy go away, no TC, no fly.
Applied military or engineering judgment is great but if being true to good old Donald we don't even have any idea about the unknown unknowns then what are those people applying judgment to? Gut instinct, seat of the pants?
Worse still what if the assessment of the known knowns is false?