Any way to use this manually without an API?

Afraid not as such @drazon; although you can add manual rules, those are the exception, the automated rules are the default and those need the API.

You could as an alternative switch to the old (legacy) plugin, cfr. https://optimizingmatters.com/documentation/ao_critcsscom/, for which you need a different type of subscription at criticalcss.com?

hope this clarifies,
frank