An update: After hours of frustrated attempts at finding a solution, it seems like I've managed to get Nik Collection 1.2.11 to work with Photoshop 2020 by following Jeffrey Tranberry's advice over at feedback.photoshop.com. There was an additional trick, though (or maybe it was that just for me, I'll share it anyway, just in case.)
I uninstalled Nik Collection 1.2.11 completely. Again, I'm on a Win10 (64 bit).
I started reinstalling it. When it asked for the Install Location, I picked the location Jeffrey pointed out: Hard Drive\Program Files\Common Files\Adobe\Plug-Ins\CC (which, in my case, was C:\Program Files\Common Files\Adobe\Plug-Ins\CC)
Then when Nik Collection asked for the Plugins Fodler Path, I picked the very same folder, that is, Hard Drive\Program Files\Common Files\Adobe\Plug-Ins\CC
Any other folder combination did not work. All files must be, it seems, in that folder. For the time being. (And, again, maybe it's just my computer. You might want to give it a try, though. Maybe you'll get lucky too.)
As is apparent from Jeffrey's post that I've linked above, this seems like a bug that they're looking into and will likely fix sometime. I hope the official fix won't break this solution, at least not without a clear warning and clear instructions about how to fix it, though.
Despite having found this workaround (thanks to an official-seeming, temporary answer elsewhere), it would be good to receive an official response and verification here.
Thanks, and I hope this helps those of you still looking for a solution.