Sorry for the delay, but I reject the assertion that you're making here:
>but you can't expect a regulator to shoulder the core responsibility of certifying a plane. The primary responsibility is always going to be on the manufacturer because no regulator will ever have the manpower to test and verify everything nor the deep visibility into to R&D process that a manufacturer would.
In a financially incentivized market-based system where a fiduciary responsibility is built into the very underlying fabric of the corporate calculus, you cannot afford to be blind to the fact that a market actor has every reason and opportunity to stuff off every cost they can to improve their bottom line. This is why we need regulators in the first place, due to opposing optimizations between the interests of shareholders/executives, and the public.
The FAA as a regulator must be capable of requesting and having delivered any piece of information relevant to the goal of airframe certification. It is the job of the manufacturer to satisfy the regulator as to the objective safety of the plane, and it is the regulator's job to ensure nothing is left out for expediency sake. When regulator's start talking about streamlining things for the regulated, I start to get worried.
To go into more detail, no; I do not see the FAA adopting the actual physical task or logistics of testing a plane; however, I do see them as the final authority in terms of "Is the design complete" and "is your testing sufficient?"
This means that an Engineer, free of the inherent bias that comes from being dependent on the manufacturer for their paycheck, and acting in the public's interest as an external agent, needs to be as fully briefed on the entirety of the operating and physical details of every plane. It doesn't need to be the same person with it all; but the point is between the FAA as an external agency, and the manufacturer, there should be two independent agencies with enough understanding of the product that it can be demonstrated the manufacturer has done their due disclosure in informing the flying public of every facet of the aircraft's behavior that nothing like the MAX boondoggle should ever even be considered as being a reasonable course of action ever again.
You had people inside Bboeing who couldn't understand why MCAS was the way it was. Given that, it is evident that the most important stakeholders in being fully informed (buyers and operators) as a consequence were also not informed before regulators cut Boeing loose to sell on the market.
In 2018, legislation was passed that made it even more difficult for the FAA to exercise it's purported authority so long as a corporate representative assured them the issue was being handled internally.
I do expect anyone in an oversight position to be capable of observing things within their purview; and in terms of evaluating designs, the tangible nature of the principles and forces involved with aviation should be conducive to clear communication and reproducibility between the manufacturer and the regulator. The difference to a business in a functioning regulatory regime is that the manufacturer should see it's job as revealing new ground to a regulator, and leveraging the regulator as the source of of friction that peels away any uncertainty from the design. This can only happen in an environment where a "no more secrets" approach to business is maintained.