Jamstack is a spectrium. At one end you have a folder of HTML, CSS, and image files a webserver serves without any extra processing. Most people would agree this is a static site. Things start to get into more of a gray area when we start adding serverless functions, external calls to an API and SPAs.
For me, the keyword is pre-rendering. If the majority of content and interactions comes from a static file served by the webserver, it's a static site. If there is on-demand processing or large portions of content is coming from an API, it's a dynamic site.
To your questions:
Yes, this could be an embed on your page using something like https://graphcomment.com/ or an API that you set up on your own server.
Yes, this is why Jamstack/static sites are gaining popularity. You have all the benefits of a static site - they're fast, easy to scale & simple while have dynamic portions of the site where needed.
It depends on what you're trying to do. Is the content mostly static with some dynamic elements (e.g. a blog with comments) or is everything dynamic (e.g. Twitter/Facebook feed)? ECS is a good option if you're creating a dynamic application. You can serve static files from ECS and put a CDN like Cloudfront infront of it to deliver them quickly.
Guarda, ho provato giusto adesso (proprio avendo letto questo tuo commento) dopo un bel pezzo che anch'io ne avevo notata la sparizione, e be', sono appena tornati! Basati su questa piattaforma, mi sembra che abbiano ancora un po' di problemi: probabilmente stanno mettendo a punto il sistema... :)