You can easily add them by following the instructions on their site.
On immutable fedora it can be done via
curl -o - https://repository.mullvad.net/rpm/stable/mullvad.repo | sudo tee /etc/yum.repos.d/mullvad.repo
rpm-ostree uninstall mullvad-vpn --install mullvad-vpn
# after reboot, if not working
sudo systemctl start mullvad-daemon
Does your network not support UPnP? You shouldn’t normally need to port forward in order to seed a torrent, unless your network prevents NAT traversal.
Uhm, I dont know? Does that work over Mullvad? I thought then it only works if the other person has an open port