@pfaffman you’re a genius! It worked!! 
Following Setting up Let’s Encrypt with Multiple Domains, the re-building Discourse followed by rebuilding mail-receiver (one of the two probably wasn’t necessary), did the trick. Voila!
Later this evening, I’d like to copy/paste the “Straightforward direct-delivery incoming mail” OP into a Google Doc and offer my suggested edits to that. At no fault to any of you, there are parts of the OP that are not intuitive to someone like me who doesn’t know or understand command-line stuff. I think the two goals I defined above are very common for many installations, and now I know the end-goal can be accomplished.
I have other suggestions for the entire setup tutorial. Have a running list of suggested edits to make the process as simple as possible.
Thanks so much for helping me along here! Super appreciated!