Yes, the adapter and the Mini are running 5V.
Last night, I was thinking about this and seemed to remember that the PL-2303HX chip was yet another one subject to "driver wars" to make clones of them inoperable, though I believe the clones installed "properly" with the later poisoned drivers. Driver rollback to an earlier version was required to clear this problem.
Just because Device Manager says something is installed doesn't guarantee it actually works.
If this is the case, the symptoms here would be explained.
A loopback test would also reveal this, I imagine. A search of "Serial Monitor" gives me a lot of different stuff; which program should I use for this?
Thanks for any guidance on this!