The only option you have with external CSS resources with AdvAgg is to move them to the TOP of the page. You could move the resource to the bottom of the page via a hook_alter_css in a custom module. But seeing this is the bootstrap CSS I can guarantee that will render your website broken.
Unfortunately, you cannot do anything about the CSS file with Pagespeed. You could host the CSS file in your site, but then you would lose the advantage of the CDN, which I see that the rest of your site is not using a CDN. Remember that Google Pagespeed scores are only one of many benchmarks and tests you should use in making a website perform. It is best to compare many different sources of data for optimizing your site (Pagespeed, YSlow, Pingdom Page Test, Webpagetest.org, etc.) And with optimization you have to give and take, it is nearly impossible to get a perfect score because you have to make sacrifices in certain areas. Being this CSS file is on a CDN and it is minified already, the impact to your site is honestly low. If you were to move the CSS to your site so AdvAgg could inline it, it would actually be worse for your site. It is a large CSS file that is critical for rendering your website, a CDN is the best place for it! I personally have found Pagespeed to be unrealistic in it's scoring from time to time.