Not who you're replying to but here are some answers:

1. For internet accessibility there will be things to consider. Do you want non-techy friends or family on it? If so you may need to port forward or use a vps. Some ISPs don't allow port forwarding, but there are a million solutions to this. Otherwise you can use a VPN to connect to your network and have access to your server that way.
2. Old laptops are great for hosting. They are typically more power efficient than desktops, although the expansion options aren't great if you don't like jank.
3. Not sure.
4. Yes and kinda. It is super light unless you're running it on really old stuff. It is easy once you learn. I recommend docker compose to keep everything in a single config file. You can use watchtower to keep things automatically updated.
5. I like Debian. I too use a rolling distro on my desktop, and some people do like rolling distros for servers. The idea of stable release distros like Debian are that they are consistant and you don't have to worry about updates breaking things. The idea of rolling distros on a server is that you will only ever have to deal with small updates rather than giant ones that may break everything (like between Debian versions). I would not recommend manjaro on a server, or at all really. They have a habit of holding back packages for no real reason and causing dependancie issues. If you like rolling I would try Tumbleweed or Arch with Archinstall. There is also Endeavor OS which is basically easier Arch.
6. Letsencrypt is for https certificates. You can use self signed certificates, meaning they are made by your computer rather than letsencrypt's servers, but you will get warnings in your browser and app interfaces for your services may not work. They are just as secure for personal use. Don't do http on the internet. That is bad. You know the VPN ads that say hackers can steal all your bank passwords if you don't buy their VPN, well that is not true because your bank uses https. If you access your services through http over the internet, someone will get your password eventually. Do whatever you want on home network though.