I took a look at the WaW steam page, to see what the reviews are like and it seems as though co-op should still be functional. People who have recently purchased the game have praised the co-op element so you should be able to play co-op and multiplayer. You may have to manually search for servers, as I'm unsure as to whether the game still has built in server support from the developers.

Yes, anything co-op does not increase your online exp

There is a patch which allows you to do this... I tried it ages back but it did not work for me, you might have better luck. Note: It is definitely not official, I would check and double check with an antivirus tool and Google the file name to see if anyone else has it working.

No, you do not need Internet to play WAW Zombies. However, you do need Internet to play Multiplayer in general (unless you can find a split-screen mod). When I say Multiplayer, I mean the Multiplayer game mode and WAW Zombies with other people online. Edit: I have first-hand proof that you do not need an Internet connection to play WAW Zombies. When I play WAW Zombies, I don't need Internet unless I am playing on Multiplayer Modes (zombies, team deathwatch, campaign co-op, etc..).

I played this with a friend a few years back. I believe the way we did it was by inviting to a lobby from the friends list in-game. If that does not work, you may have to port-forward your router in order to get your friend to join.

You do not get any custom zombie maps no, however you CAN download them and put them into your game. If I recall correctly as well there are 2 zombie skins that were available in the game itself if you had a certain purchase. However if you want the entire zombie experience*and you have the money) you will have to buy the Zombie package which costs about 170$ it includes all the zombie stuff from all the CoD games that had zombie modes black ops, World at war etc... If you want to get the custom zombie maps here https://steamcommunity.com/groups/Imlikingmyzombies This should give you a head start. Hope that helps

I don't think so. Like any program, CoD WaW only gets inputs if the window is the active one -- regardless if you play with a controller or not. In the end the controller just sends key strokes to the game, thus resulting in actions in the game. So if the game loses focus, the key strokes sent by the controller just are received by the focused window. As far as I know multiple windows can't be active at the same time.