Just answering off the cuff here, but when I designed and built my second ultralight about 18 years ago (AUF), the 'inspector' (who was actually a CASA mate!) watched me do the inspection, advising if he thought I missed something.
The idea was that the inspection was overseen, but the final responsibility rest with me, the builder.
The SAAA system had an out side inspector taking all the final responsibility for your work!?
As for,
When talking about 'amateur built' ultralights (19-xxxx), they are not certified, that's the whole point.
If someone decides to improve (why would you make something worse?) a plan/kit/finished homebuilt, then it is the designer/manufacturer's prerogative to diss-own the final product.
Part of the amateur concept is learning about the technology, design, hand skills and aerodynamics of aircraft, if you just follow someone else's plans/kit/build, then you miss a lot of learning and just end up with ownership of someone else's aeroplane with your paint job.