Hi Erik,
Thank for posting this! I went through the steps you outlined and was able to get Font Awesome fonts to render correctly here at Axure HQ–sweet! I did notice one thing that hopefully makes things easier:
In Step 4: CSS URLs, when visiting https://fontawesome.com/start, you can click “All”. This will make it so that in Step 5: Add Fonts, you only have to link to one .css (all.css) and also makes it so that you don’t need to add font mappings as described in Step 6: Add Font Mappings.
Hope this helps!