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. Im have been using a seedbox from ultra for about 2 years now, had no connection issues so far. Had ratios of 20+ on some torrents that i kept seeding

Report it on ublock origins github, gets fixed pretty quickly from my experience