I'm going to reluctantly dip a toe in here....
I suspect the issue here is not one of eliminating all risk, but rather one of understanding the likely risks and their likely occurrence relative to how the aircraft is operated.
No-one is saying the aircraft is inherently unsafe - the millions of flying hours in USAF service suggests otherwise. However, that is NOT the same as understanding the risk. Nor are they suggesting that the three UK frames are somehow inferior - they're not AMARC regenerations, but rather direct KC135R conversions.
The issue for RJ appears to be that either Design disclosure for the original frame design is incomplete or inaccessible, or that the staff in MAA are either not competent or have not been sufficiently resourced to do the design assessment in time to meet the programme RTS. It's all coloured by the aftermath of H-C and people's experience / perception of the way the system appears to have been broken.
Tourist is entirely right in the idea that you cannot remove all risk from flying (especially military flying). However by the same token, a competent organisation ought to be able to understand the risks it is taking and certainly not attempt to suppress / subvert that process.
The first part of that last sentence requires a well-staffed competent engineering organisation and the last part indicates that at least some of the engineering oversight function (potentially the MAAIB) ought to be independent from the operator.