tThe lead probably came from infant/toddler nibbling on painted wooden cribs made in China, and fishing sinkers-
Impossible to prove from whence came the lead.
Not true at all. High resolution mass spec can easily do so. Different batches of lead have significant differences in isotope ratios, depending on the parent radioisotope that the lead is derived from, which ultimately goes back to what the geochemistry of the lead mass source. Likewise, recycled materials can have similar (but more subtle) differences.