Go figure. After multi-hour runs at this on two separate occasions...as soon as I posted here I seem to have found something that did the trick.
The comment from dmatyukhin in this post: [Solved] Clone arduino Mega 2560 - no serial port DCcduino Mega 2560 - Installation & Troubleshooting - Arduino Forum
Check out that post for the full details, but it did boil down to (I think) both having the CH340G/CH341 driver installed, running a command in Terminal (sudo nvram boot-args="kext-dev-mode=1"), and then restarting.