"How a manufacturer would manage lot-to-lot variation of component parts, in a third-party testing of component parts regime, to ensure finished consumer products are compliant."
In this instance, supplier provided testing is an advantage over unit testing. For example, if a manufacturer only tests one unit, yet that production run consists of 3 lots of components, then presumably only one of those lots will be tested per sku. But if the supplier of that component has an on-going testing regime, then those results would be superior to unit testing. If a supplier consistently chooses the same materials to produce their products, then that lot-to-lot variation will be insignificant. The same would apply to the manufacturer.