I think my next move (after replacing them with some decent quality parts, or even standard arms and castor correction swivels) would be to take the faulty component to a qualified fabricator and ask them what they think. A written report stating that they're not fit for purpose would be a good starting point if the supplier continues to fail to reply.
I don't think it would be unfair to name them either. Whatever they do now, there is a fair chance that other people are driving around with potentially lethal parts made at the same time as yours (if it's a bad batch of steel, or bad welding, or whatever, it is unlikely to only happen to one part). They should be warned before something goes horribly wrong. If the supplier comes back with a solution you're happy with then you can mention that too - I think anyone else with parts from the same batch would be justified in asking for theirs to be checked with X-rays or ultrasound and replaced if not up to scratch.
They have already made the error, what matters now is how they deal with it.
