

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.
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.