1: Avast! ( sounds familliar...)
2: Mostly not

3: Years ago I played JJ2 on multiple computers and on my oldest (windows 95) and the only one not having Norton Antivirus at the time I was able to only host, on the others I CTO'ed and also couldn't host.

4: I can join all servers.
5: I only play JJ2 on that computer.

6: there are some active people in the server (you can see them in F9 list and you can see them chat) I mostly have these 2: A) You can see other players in the server and they can see you. You still time out after 30 seconds.
B) You can see other players in the server, but they cannot see you or they see you as if you were timing out (blinking). also I sometimes recv things and still timeout.

7: Still only exposed host (DMZ) works for me totally.