I might be wrong, but adding multiple styles for one Font-Family used to work in the last month or so!
Because I had multiple styles (bold, italic, etc.) for my local Oswald font, but now when I go into Fontbook again, it only shows one style and the drop down is not enabled in the Page Builder.
Via chat I was told by support staff to not use Font Squirrel but another page (transfonter.org iirc), but when I want to upload the same zip again, I get an 500 error.
And no, adding each style (bold, normal, italic) as single font AGAIN is just not an option for me, as I would have to edit AGAIN my pages that are using them already.