Yes, is possible and not that hard, you add the torrent back into your client pause it (or dont even let it start downlaing) and point it to the already downloaded files by changing the download folder, after that just verify the files in the client, it sould see the already downloaded files and verify that they are the same, once that is done you can continue seeding.
its your standard torrent tracker, works like all the public ones listed in the megathread you just need to login. To make an account to head to the forum and create on there, they have a post about how to apply for an invite thats not hard to find (just scroll down on the homepage and you should find it). An admin will respond to you and will let you know if you got in
For FTP the only one i know is fxp-terminal
Torrent sites are pretty much all private, but immortuos allows for applications on their forum. There are more sites that take applications but they are imo not nearly as good as immortuos
If all you care about is bypassing the paywalls then i suggest using BPC, its completly seamless. If you dont want do install an extension then you can use archive.today. The site you linked is probably safe but why use something unkown when there are trusted alternatives.
Yes, you can just use the seedbox to download and run jellyfin locally with the media staying on your box, but it might be a lot easier to just get a seedbox that allows you to run jellyfin on it.