Depending on browser caching (which is why it took a while for me to see the issue

) and all that fun, the correct images on the buttons should now start to begin showing.
I've ended up adopting suggestion #1 from [member=6]QOAL[/member] as the alternative aliases which were being used in the custom copy of the fontawesome css are there to map things from the SMF forum values to the desired FA icon (so it was relevant as the hint helped me to work out why I couldn't find what I was looking for between the aliases and the theme's code

).
And as it's just the CSS that needs to be tweaked I can still leverage the CDN cache for the font files, etc whilst maintaining a locally provided CSS file (which should be CDN backed anyway from using Cloudflare to handle things).
-dro