I believe my initial description already answered all of your questions — I’m using the Custom Headings module and Questrial IS NOT LISTED in the font dropdown box although many other Google Fonts are. Google Fonts Didact Gothic is not in the dropdown either.

You can see what I’m describing if you look at the Font Family dropdown at the bottom of the Custom Heading settings box, under the General tab.

I didn’t do anything to add all the Google Fonts that already appear there — as I assume you know, they are automatically listed as part of the theme install.

My question is what’s the best way to use a Google Font when it’s not already listed in that dropdown?