[somenameforme]

There is no exact and objective set of hoops one can jump through to prove a sufficiently complex (let alone novel) technology safe within a certain bounds, short of doing exactly what it will be doing over a large sample, which is often not economically feasible. In fact one of the first things that happened early on in the Apollo program is that mathematical risk modeling was completely scrapped. The results were always so pessimistic that NASA found it impossible to move forward with it!

So this leads to judgement calls from NASA that are opaque and, in practice, are not necessarily grounded in safety, as per your own example as well. NASA clearly did not want SpaceX doing propulsive landings and was making sure to dot all their i's and cross their t's with them, while simultaneously going YOLO with Boeing and actively greenlighting their vessel which clearly was not even remotely safe for a human. In this context, it's highly unlikely SpaceX could have convinced NASA to more forward with the propulsive landings, even if they were the safest thing ever invented.