In practice, many of the files we're seeing this issue on are binaries (image files) and the people doing conflict resolution are inspecting each file and using P4V to select which one to keep - so unfortunately can't rely on them running a specific undocumented resolve option on the CLI
I'm also understandably wary of messing with server-wide configs that could affect other projects, regardless of how much I still think dm.integ.tweaks=2 should be true for most cases.
But both of these led me to what looks like the silver bullet I was hoping for: the integration script pushing from Authority to Outsource simply has to use the undoc "integrate -Znnn" option:
p4 integ -S //stream/Child -r -Z2 p4 resolve -asand truly unresolved conflicts are still flagged, while anything resolved in favour of either parent is synchronised without issue.
Thanks for your help!