YES! Son of a gun!
Okay, the correct pathway to fix this error in my version of Windows 10 (found with your help) is NOT to use File Explorer to view the font in the Fonts folder or the Control Panel, OR to search the font name itself (it stalls out) but to re-download the font, EXTRACT IT to the Downloads folder or elsewhere, but NOT to the Fonts folder, and DO NOT obey Windows' prompt to "Install?" but right-click on the font file only after you have extracted it to its own folder. THEN (and it looks like ONLY then) you get the menu option for "install for all users." I don't know whether I need to install all new fonts via the back door like this to get them to work with paint.net, but I'm going to anyway.
Manually OKing the font for all users and programs via the Control Panel doesn't do it, and right-clicking before extraction or in the Fonts folder doesn't either, and Windows 10 offers you multiple options to do it "wrong" (which seem to have been "right" for all the other programs I'm using). I'm so glad that menu option IS available. You just have to follow a very specific path.
THANK YOU!