I have built a few trailers over the years here. Inspection here is pretty straight forward. There is an inspection process but it basically ensuring the coupler, axle
are correct. Nobody looks at the design engineering or does welding inspections.
If I were approached by a commercial transport (Highways Inspector) he would see my approval stickers and wouldn't have anything to discuss...
To Cover my Axx I also had a certified welder / fabricator inspect my welding and he signed off on my build.
We are not quite as litigious in Canada as the USA so do your research.