Solved!
Thank you, everyone.  (For the record, I did run as admin, and I did try putting the .dll folder in all kinds of places.)
Here's what worked.  The patient tech supports at IK Multimedia suggested (among many other things previously) to open my windows registry editor and manually remove anything named Modo Bass from the HKEY_CLASSES_ROOT folder (I act like I know what I'm talking about - I do not, but they spelled it out very simply).  Also, they had mentioned previously to install visual c++2015, which was a pain I wish on no one, but finally got it to work after deleting c++2017.
Anyway, I don't know if it was the former or the latter, but I finally saw Modo sitting there in my VST3 list.
Only thing is, as a VST3, I only get left channel, so I use it as VST and it's fine.
Thanks, all, for your support!