CS
Your choice to flag up the Vans aircraft as amateur built is a bad one the only thing "amature" about the Vans is that no one is paid to build them, it is for all intents and perposes a factory built aircraft assembled in shed & garages all over the world and made out of standard aviation parts.
On the other issues I think we are in violent agreement, when I say quality control I mean just that, it is not nessesaraly reams of paper it is just the assurance that the part is fit for service, as you say motorcycles don't fall apart but we must be sure that the HTS bolt is fit to hold the motorcycle together and is NOT the bolt that held together the packing case the motorcycle that the motorcycle was delivered in !
In a lot of cases for American size standard parts buying from normal aviation sorces is the cheapest way of quality assurance, for metric the best way to go would be a reputable sorce who can provide a certificate of conformity.
As for the CAA's attitude to single seat microlights, I think they are relying on the fact that these don't have much energy and the only person likely to be killed by the machine is the pilot........... Added to the fact that the CAA scale of charges is regulated by the MTOW resulting in there being very little money in regulation of microlights.