internet sharing (mojave) AND two-way connection
Answering my own question for the record if anyone else searches. FYI I had to go to a full router setup to get this to work. The new configuration is:
wireless router &lt;---&gt; MBP &lt;---&gt; eth &lt;---&gt;|    |&lt;---&gt; eth &lt;---&gt; PC
                                         router

The MBP side of the router is the &quot;internet&quot; connection and the other side is the 4-port hub provided by the router for its private local network. Had to set up port forwarding on the router to target individual PCs via RDP, and there are Raspberry Pis on the hub side as well, but all is functional.
I still think I'm lacking something because &quot;internet sharing&quot; is inherently 2-way on some level, but I don't know enough about Apple's implementation of this sharing to know what's missing.