Okay, so I have fixes for both issues in a working state now Not sure it'll be done in time for 3.3 though
https://github.com/NodeBB/NodeBB/pull/11820
If this is merged, other than fixing the issue with the icon selector, NodeBB will have support for FA pro enabled by installing the npm package (see instructions here: https://fontawesome.com/docs/web/setup/packages - this step requires setting up access to their package repo so it can't be done by NodeBB) and adding a configuration value (two if you want additional styles - FA seems to recommend not including all styles at once, so NodeBB will just use solid, regular and brands by default. Others require adding to configuration and rebuilding css)