For 1 I would be inclined to use clustering then reverse proxy the subdomains to categories. Rather than spooling individual instances for each. I've done this at my own site.
dnb.35hz.co.uk will take you to the electronic category.
You could then make a few changes to how your homepage/main website looks and do it that way.
As an additional. Anything you're not sure exists, drop an email to sales [at]nodebb.org and they may be able to create what you need.
Here are steps on customizing Font Awesome. Mayhaps these will help.
Thank you, I'll have a look. Would any updates of the forum affect Font-Awesome in future? Ideally, adding the icon in a way which wouldn't require updating the icons each time would be the best way forward.