UPDATE (& TL;DR & Introduction!): Killing Font Book between operations avoids all sorts of errors and weird display issues you'll get if you try to install multiple font files in a single 'session'. Open Font Book from fresh, locate font files in Finder, drag them into it and you'll do everything in one operation, and it will even update to show you what's happened!
> Like what?
Errors about duplicate fonts. Several occasions where dialogs open and close immediately, indicating some kind of problem, but not sure what. Even after going through the "yes, please do really install this font, even though there are 'duplicate fonts'", nothing seems to happen sometimes.
I've managed to install three of the fonts (font variants?) — Regular, Bold, and Bold Italic — but Italic refuses to install altogether.
> How is that in any way "horrible"?
Having to open four separate font files, then click a button each time to actually install them seems like a pretty tortuous process — I'm grateful it was only 4! It would be a LOT nicer if I could just select the files, right-click and choose "Install font(s)" and have it all done in a single step. That's totally aside from all the errors.
Having said that ... pro-tip: drag-and-drop. I always forget that drag-and-drop is sometimes the 'first class' way of doing something on OSX. So the quickest way, I think, would be:
1. Open 'Font Book'
2. Locate files in Finder
3. Drag and drop from the latter to the former
This process finally allowed the "Italic" variant to be installed. I'm assuming it will work with multiple variants at once; I would test that, but trying to remove the family/variants in Font Book just gives me a 'no-op' (after a confirmation dialog).