Definitely recommend using port forwarding for seeding. But seems like you want multiple ports. Have to make sure the provider you use allows that. Alternatively you could use VPN to port forward your torrent client and cloudflare tunnels to allow access to Jellyfin
From what I heard content can get taken down there, not the best for niche stuff, not the best for new releases (have to wait 1-2 hours from when something is already on privates within minutes)
There used to be a browser extension to dump the key to your browser console. May not work anymore. It should be possible go get the key from android phones (might need to root it) good luck but it may be just easier to grab it from a private tracker or make a request for it on one
Depending on the hosting/ddos protection of the site in question they cannot do Punch 2. If they are using Cloudflare for example then the ip address is shared with a bunch of other sites.
Once it's released on a streaming service, quality private trackers will have it within minutes.