If you have a listen port set and rule applied to firewall then UPnP or NAT-PMP are redundant I'm behind a NAT, with no port-forwarding configured on it, so I would say UPnP or NAT-PMP are essential in order to be able to accept incoming connections. And I'm also interested in the seeding strategy ...

Does Deluge Torrent actively try to upload seeding torrents? More specifically, does it fetch a peer list for torrents that it is seeding, pick some of the peers and contact them, offering to send some pieces? Or, it only passively waits and listens if somebody else takes the initiative and connects...