To prioritise above-the-fold content, you’d analyse all your CSS rules and inline only those rules that affect the styling of the page above the fold. All the other CSS would be put in a separate .css file, which is then loaded after page load.
If the script cannot be deferred, then you shouldn’t defer it. Or you could, but then you might end up with an ugly flash of content or something broken on the page. But that’s not worth the hassle of a somewhat quicker website.
If you want to optimise your page load time, I’d not look at the two points you mentioned above. You can make much bigger gains by dropping the Google fonts. A whopping 75% of your page size consists out of fonts, and they use 31% of the HTTP requests.
In comparison, your CSS is 12kb while the fonts are 476kb (see the speedtest link below for where I got those values). So CSS optimisation is just a tiny part of the page.
By the way, here’s a website speed test I performed on your site: speedtest.