Is it considered acceptable to repost movies or TV shows from Usenet that are not available on any open torrent tracker? Obviously, the reposted content will be left unchanged and will have the exact filename as originally uploaded by the initial uploader.
You can though it's a bit of a roundabout way of doing it.
P2P releases typically come from private trackers, so you're having them go from private trackers --> usenet --> public/private torrents
Scene releases that leak to the public typically hit private trackers/usenet around the same time, so you're having those go scene --> private trackers/usenet --> public/private torrents
In others words anything you're seeing in usenet has already been uploaded to at least some private trackers & possibly public torrents.
Of course with public torrents anything goes, unfortunately with the demise of RARBG public torrent users are only seeing a fraction of scene/p2p releases. 1337x/TorrentGalaxy does cover some of this but they aren't covering nearly as much as the RARBG uploaders used to. So IMO if you're seeing a scene/p2p release that hasn't already been uploaded at 1337x/TorrentGalaxy then sure go ahead & create the torrent from your usenet download.
Unclear what you are talking about, you mention Scene but also quote my comment about P2P. Which one did you mean? They are totally different.
The scene doesn't do anything with torrents/usenet/etc., they are all about their own FTP servers. Yes, other people do have access to them & then spread those releases over to torrents/usenet/etc. but that's not the scene doing it.
P2P are the internal release groups at private torrent trackers. Most of them are not interested in uploading anything to usenet/IRC. Other people do grab those releases & then spread them elsewhere e.g. usenet/IRC.
In this case, the indexer is talking about the .nzb, which does usually contain an identifier about which account accessed it. Remember that an .nzb is just a plaintext file providing links to the many split .rar files that will be combined during unpacking, plus the parity files that can be used to recreate missing .rar files if one is missing.
Indexers want to prevent the .nzb from being leaked onto the open internet, or to other indexers, to prevent those links from getting a DMCA/NTD takedown request for as long as possible.
There is no way to identify who downloaded a .mp4, they are not fingerprinted & every person who downloads is getting an identical copy. Share them widely.