That link was extremely helpful! My unsubstantiated belief is that the process-shutdown bug was new in Windows 10 Anniversary Edition, and that link supports that idea. It sounds like Windows 10 Anniversary Edition changed the management of GDI objects to avoid leaking kernel addresses and that made HmgNextOwned more expensive, thus triggering the visible serialization of process destruction.

Cool!

I really wish I could upvote the parent post more times. Failing that I've updated the latest post to link to it as an explanation for how the whole problem got started.