1. AFAIK, not out of the box. The Tiny Editor appears to have a footnote plugin, but I have zero idea how to implement this. Here’s the link in case you’re more tech-savvy than me: https://www.tiny.cloud/docs/tinymce/6/footnotes/
2. As of now: there’s still no simple solution. There are some recent posts floating around in this forum and in the GitHub of people who have found ways of modifying post templates and headers to accommodate multilingual posts in one site, but there’s still the question of menus and whatnot. I saw someone in the Forum describe creating separate directories for each language, but it seemed very tedious.
3. Are you asking for ad placement suggestions, or asking how to implement the ads space like in Technews, Editorial, Easyblog, and N-tech?
4. It’s not super well known yet, so you probably won’t make much, but give Web Monetization a look: https://webmonetization.org/. It can be implemented on static sites, and there are some interesting demos on gated content: https://webmonetization.org/docs/exclusive-content
Otherwise, you could try integrating something like Authorizer with payment processing (if you have the skills for it), or pay for a service like Userbase, which can connect to Stripe (still requires the skills to implement).