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

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

the megathread has good sources, i personally use lucida

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