What I'm wondering is -- although an item would be found to be legally considered public domain in all applicable jurisdictions when examined by law clerk -- what are the practical rules for passing copyright clearance checks/challenges by Amazon, Kobo, etc. for public domain art.

You don't want a book's carefully choreographed launch sabotaged by various companies' Kafkaesque bureaucratic processes, while you're trying to finesse the algorithm and the publicity you've scheduled.

(I should've said upfront that I know a bit about public domain and copyright law as a layperson. And I also know that I can't necessarily point a CSR to law surveys if something gets flagged. I need to know the "do X and avoid Y" that is proven to actually work smoothly with all these companies' ideas of "copyright clearance", so that I hopefully never have to talk to a CSR.)