If the designers are correct, then the regulator should accept that
In between those two is the demonstration and finding of design compliance for certification. The "applicant" (in this case Boeing) desires that their product receive FAA certification. When the applicant, with uniform intent within their certification division, has determined that design compliance is ready to be demonstrated, they "declare" this to the regulator, who then may review (read/witness testing/review test data) the demonstration of compliance, and then within their scope, find that design compliance has been demonstrated. When there is a record of a finding of design compliance for every affected design requirement, design approval will be issued.
Much of the foregoing process may happen entirely within an applicant's delegated organization, and be subject to surveillance oversight, Or, the FAA may hold the findings of design compliance to themselves, in which case, the applicant waits. The regulator should only accept what they are confident about in within the terms of delegation. Otherwise, the regulator acts on behalf of the taxpayers to directly serve the applicant.
It is a reality that in our increasing technology world, there are more "novel" features being presented for certification, and in some cases, the existing design requirements do not embrace the new technology (electric power aircraft are certainly an example of this). In such case, the FAA and industry must work together to create new design standards, to which the novel design may be found to comply. When this happens, it is very likely that the knowledge base for the novel feature will reside much more with the applicant, than with the FAA, who will still be playing catch up - as they did not invent it. So it's a difficult balance for the FAA to strike to satisfy the industry by encouraging innovation, and creating new design standards to be met, yet satisfying the taxpayer expectation of bulletproof design standards to which the new feature may then be found to comply - without adding hundreds of new staff to the public payroll.