It functions without port forwarding, but it’s much better with. While I can’t speak for the Java client, i2pd has support for UPnP, which might make things easier in some environments. You also don’t need a VPN for I2P, since the network takes care of the anonymising

it’s generally advisable to not run a vpn in front of i2p. it will run better without one and you introduce another point of failure that can reduce your anonymity in some circumstances.

Can’t answer that sorry, i barely tried i2p

Does p2p over i2p require port forwarding?

No, you’ll torrent fine via I2P without port forwarding. Note that the torrents are running through the I2P network so technically you wouldn’t want to open your torrent client to the clearnet anyway. It’d be like purposely introducing a VPN leak in your VPN setup by allowing it traffic outside the VPN (or in this case I2P).

Been a bit since I’ve tinkered with torrents over I2P but for a while I was seeding torrents over I2P and would get pretty good seed/upload speeds to other torrent peers. Was mainly testing with i2psnark and BiglyBT.

Fun fact: Torrent hashes don’t change, so that same exact torrent you might download at TPB or wherever would still download within I2P as long as there’s someone seeding it there.

Also see https://geti2p.net/en/faq#ports

Not exactly what you’re asking but you can open a port forward for I2P itself to better communicate with other I2P routers. “routers” in this case usually means other people running I2P.