Does anybody know if the functionality mentioned in the "One final note" — having all children, grandchildren, great-grandchildren, etc. of a designated "main" process reliably terminated when that "main" process terminates — is available via something more lightweight than the containers?

Traditional process groups/sessions are laughably inappropriate for this purpose since breaking out of them is trivial and in fact, about half of applications that internally use worker (sub)processes do exactly that, in order to (re)implement their own job control.