I probably should have said DMARC and not DKIM (I was spam combatant policies all together as if they were all the same/similar, but I should not have done that; too many "new" acronyms to keep straight).
Thank you for providing the screen-shot showing List Settings/Settings/DMARC Mitigations with "DMARC mitigation action" set to "Replace From: with list address" and "DMARC Mitigate unconditionally" set to "Yes".
I tried this and it cleared up the problem (at least with one sender tested so far). I also noticed the my colleague was already using these settings in a list he manages on the same server.
Unfortunately, now when I respond to list emails (with the re-written From address) my (very old) email client (I still use Mulberry!) defaults to responding to the sender instead of the list, as it had before. I'm hoping this is a nuance of my client and isn't something the other members are going to have to grapple with.